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Fixed MTP to work with TWRP

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Guidance for writing policies
=============================
Try to keep transactionality out of it. The core is careful to
avoid asking about anything that is migrating. This is a pain, but
makes it easier to write the policies.
Mappings are loaded into the policy at construction time.
Every bio that is mapped by the target is referred to the policy.
The policy can return a simple HIT or MISS or issue a migration.
Currently there's no way for the policy to issue background work,
e.g. to start writing back dirty blocks that are going to be evicte
soon.
Because we map bios, rather than requests it's easy for the policy
to get fooled by many small bios. For this reason the core target
issues periodic ticks to the policy. It's suggested that the policy
doesn't update states (eg, hit counts) for a block more than once
for each tick. The core ticks by watching bios complete, and so
trying to see when the io scheduler has let the ios run.
Overview of supplied cache replacement policies
===============================================
multiqueue
----------
This policy is the default.
The multiqueue policy has three sets of 16 queues: one set for entries
waiting for the cache and another two for those in the cache (a set for
clean entries and a set for dirty entries).
Cache entries in the queues are aged based on logical time. Entry into
the cache is based on variable thresholds and queue selection is based
on hit count on entry. The policy aims to take different cache miss
costs into account and to adjust to varying load patterns automatically.
Message and constructor argument pairs are:
'sequential_threshold <#nr_sequential_ios>'
'random_threshold <#nr_random_ios>'
'read_promote_adjustment <value>'
'write_promote_adjustment <value>'
'discard_promote_adjustment <value>'
The sequential threshold indicates the number of contiguous I/Os
required before a stream is treated as sequential. The random threshold
is the number of intervening non-contiguous I/Os that must be seen
before the stream is treated as random again.
The sequential and random thresholds default to 512 and 4 respectively.
Large, sequential ios are probably better left on the origin device
since spindles tend to have good bandwidth. The io_tracker counts
contiguous I/Os to try to spot when the io is in one of these sequential
modes.
Internally the mq policy maintains a promotion threshold variable. If
the hit count of a block not in the cache goes above this threshold it
gets promoted to the cache. The read, write and discard promote adjustment
tunables allow you to tweak the promotion threshold by adding a small
value based on the io type. They default to 4, 8 and 1 respectively.
If you're trying to quickly warm a new cache device you may wish to
reduce these to encourage promotion. Remember to switch them back to
their defaults after the cache fills though.
cleaner
-------
The cleaner writes back all dirty blocks in a cache to decommission it.
Examples
========
The syntax for a table is:
cache <metadata dev> <cache dev> <origin dev> <block size>
<#feature_args> [<feature arg>]*
<policy> <#policy_args> [<policy arg>]*
The syntax to send a message using the dmsetup command is:
dmsetup message <mapped device> 0 sequential_threshold 1024
dmsetup message <mapped device> 0 random_threshold 8
Using dmsetup:
dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \
/dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8"
creates a 128GB large mapped device named 'blah' with the
sequential threshold set to 1024 and the random_threshold set to 8.

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Introduction
============
dm-cache is a device mapper target written by Joe Thornber, Heinz
Mauelshagen, and Mike Snitzer.
It aims to improve performance of a block device (eg, a spindle) by
dynamically migrating some of its data to a faster, smaller device
(eg, an SSD).
This device-mapper solution allows us to insert this caching at
different levels of the dm stack, for instance above the data device for
a thin-provisioning pool. Caching solutions that are integrated more
closely with the virtual memory system should give better performance.
The target reuses the metadata library used in the thin-provisioning
library.
The decision as to what data to migrate and when is left to a plug-in
policy module. Several of these have been written as we experiment,
and we hope other people will contribute others for specific io
scenarios (eg. a vm image server).
Glossary
========
Migration - Movement of the primary copy of a logical block from one
device to the other.
Promotion - Migration from slow device to fast device.
Demotion - Migration from fast device to slow device.
The origin device always contains a copy of the logical block, which
may be out of date or kept in sync with the copy on the cache device
(depending on policy).
Design
======
Sub-devices
-----------
The target is constructed by passing three devices to it (along with
other parameters detailed later):
1. An origin device - the big, slow one.
2. A cache device - the small, fast one.
3. A small metadata device - records which blocks are in the cache,
which are dirty, and extra hints for use by the policy object.
This information could be put on the cache device, but having it
separate allows the volume manager to configure it differently,
e.g. as a mirror for extra robustness. This metadata device may only
be used by a single cache device.
Fixed block size
----------------
The origin is divided up into blocks of a fixed size. This block size
is configurable when you first create the cache. Typically we've been
using block sizes of 256KB - 1024KB. The block size must be between 64
(32KB) and 2097152 (1GB) and a multiple of 64 (32KB).
Having a fixed block size simplifies the target a lot. But it is
something of a compromise. For instance, a small part of a block may be
getting hit a lot, yet the whole block will be promoted to the cache.
So large block sizes are bad because they waste cache space. And small
block sizes are bad because they increase the amount of metadata (both
in core and on disk).
Cache operating modes
---------------------
The cache has three operating modes: writeback, writethrough and
passthrough.
If writeback, the default, is selected then a write to a block that is
cached will go only to the cache and the block will be marked dirty in
the metadata.
If writethrough is selected then a write to a cached block will not
complete until it has hit both the origin and cache devices. Clean
blocks should remain clean.
If passthrough is selected, useful when the cache contents are not known
to be coherent with the origin device, then all reads are served from
the origin device (all reads miss the cache) and all writes are
forwarded to the origin device; additionally, write hits cause cache
block invalidates. To enable passthrough mode the cache must be clean.
Passthrough mode allows a cache device to be activated without having to
worry about coherency. Coherency that exists is maintained, although
the cache will gradually cool as writes take place. If the coherency of
the cache can later be verified, or established through use of the
"invalidate_cblocks" message, the cache device can be transitioned to
writethrough or writeback mode while still warm. Otherwise, the cache
contents can be discarded prior to transitioning to the desired
operating mode.
A simple cleaner policy is provided, which will clean (write back) all
dirty blocks in a cache. Useful for decommissioning a cache or when
shrinking a cache. Shrinking the cache's fast device requires all cache
blocks, in the area of the cache being removed, to be clean. If the
area being removed from the cache still contains dirty blocks the resize
will fail. Care must be taken to never reduce the volume used for the
cache's fast device until the cache is clean. This is of particular
importance if writeback mode is used. Writethrough and passthrough
modes already maintain a clean cache. Future support to partially clean
the cache, above a specified threshold, will allow for keeping the cache
warm and in writeback mode during resize.
Migration throttling
--------------------
Migrating data between the origin and cache device uses bandwidth.
The user can set a throttle to prevent more than a certain amount of
migration occurring at any one time. Currently we're not taking any
account of normal io traffic going to the devices. More work needs
doing here to avoid migrating during those peak io moments.
For the time being, a message "migration_threshold <#sectors>"
can be used to set the maximum number of sectors being migrated,
the default being 204800 sectors (or 100MB).
Updating on-disk metadata
-------------------------
On-disk metadata is committed every time a FLUSH or FUA bio is written.
If no such requests are made then commits will occur every second. This
means the cache behaves like a physical disk that has a volatile write
cache. If power is lost you may lose some recent writes. The metadata
should always be consistent in spite of any crash.
The 'dirty' state for a cache block changes far too frequently for us
to keep updating it on the fly. So we treat it as a hint. In normal
operation it will be written when the dm device is suspended. If the
system crashes all cache blocks will be assumed dirty when restarted.
Per-block policy hints
----------------------
Policy plug-ins can store a chunk of data per cache block. It's up to
the policy how big this chunk is, but it should be kept small. Like the
dirty flags this data is lost if there's a crash so a safe fallback
value should always be possible.
For instance, the 'mq' policy, which is currently the default policy,
uses this facility to store the hit count of the cache blocks. If
there's a crash this information will be lost, which means the cache
may be less efficient until those hit counts are regenerated.
Policy hints affect performance, not correctness.
Policy messaging
----------------
Policies will have different tunables, specific to each one, so we
need a generic way of getting and setting these. Device-mapper
messages are used. Refer to cache-policies.txt.
Discard bitset resolution
-------------------------
We can avoid copying data during migration if we know the block has
been discarded. A prime example of this is when mkfs discards the
whole block device. We store a bitset tracking the discard state of
blocks. However, we allow this bitset to have a different block size
from the cache blocks. This is because we need to track the discard
state for all of the origin device (compare with the dirty bitset
which is just for the smaller cache device).
Target interface
================
Constructor
-----------
cache <metadata dev> <cache dev> <origin dev> <block size>
<#feature args> [<feature arg>]*
<policy> <#policy args> [policy args]*
metadata dev : fast device holding the persistent metadata
cache dev : fast device holding cached data blocks
origin dev : slow device holding original data blocks
block size : cache unit size in sectors
#feature args : number of feature arguments passed
feature args : writethrough or passthrough (The default is writeback.)
policy : the replacement policy to use
#policy args : an even number of arguments corresponding to
key/value pairs passed to the policy
policy args : key/value pairs passed to the policy
E.g. 'sequential_threshold 1024'
See cache-policies.txt for details.
Optional feature arguments are:
writethrough : write through caching that prohibits cache block
content from being different from origin block content.
Without this argument, the default behaviour is to write
back cache block contents later for performance reasons,
so they may differ from the corresponding origin blocks.
passthrough : a degraded mode useful for various cache coherency
situations (e.g., rolling back snapshots of
underlying storage). Reads and writes always go to
the origin. If a write goes to a cached origin
block, then the cache block is invalidated.
To enable passthrough mode the cache must be clean.
A policy called 'default' is always registered. This is an alias for
the policy we currently think is giving best all round performance.
As the default policy could vary between kernels, if you are relying on
the characteristics of a specific policy, always request it by name.
Status
------
<metadata block size> <#used metadata blocks>/<#total metadata blocks>
<cache block size> <#used cache blocks>/<#total cache blocks>
<#read hits> <#read misses> <#write hits> <#write misses>
<#demotions> <#promotions> <#dirty> <#features> <features>*
<#core args> <core args>* <policy name> <#policy args> <policy args>*
metadata block size : Fixed block size for each metadata block in
sectors
#used metadata blocks : Number of metadata blocks used
#total metadata blocks : Total number of metadata blocks
cache block size : Configurable block size for the cache device
in sectors
#used cache blocks : Number of blocks resident in the cache
#total cache blocks : Total number of cache blocks
#read hits : Number of times a READ bio has been mapped
to the cache
#read misses : Number of times a READ bio has been mapped
to the origin
#write hits : Number of times a WRITE bio has been mapped
to the cache
#write misses : Number of times a WRITE bio has been
mapped to the origin
#demotions : Number of times a block has been removed
from the cache
#promotions : Number of times a block has been moved to
the cache
#dirty : Number of blocks in the cache that differ
from the origin
#feature args : Number of feature args to follow
feature args : 'writethrough' (optional)
#core args : Number of core arguments (must be even)
core args : Key/value pairs for tuning the core
e.g. migration_threshold
policy name : Name of the policy
#policy args : Number of policy arguments to follow (must be even)
policy args : Key/value pairs
e.g. sequential_threshold
Messages
--------
Policies will have different tunables, specific to each one, so we
need a generic way of getting and setting these. Device-mapper
messages are used. (A sysfs interface would also be possible.)
The message format is:
<key> <value>
E.g.
dmsetup message my_cache 0 sequential_threshold 1024
Invalidation is removing an entry from the cache without writing it
back. Cache blocks can be invalidated via the invalidate_cblocks
message, which takes an arbitrary number of cblock ranges. Each cblock
range's end value is "one past the end", meaning 5-10 expresses a range
of values from 5 to 9. Each cblock must be expressed as a decimal
value, in the future a variant message that takes cblock ranges
expressed in hexidecimal may be needed to better support efficient
invalidation of larger caches. The cache must be in passthrough mode
when invalidate_cblocks is used.
invalidate_cblocks [<cblock>|<cblock begin>-<cblock end>]*
E.g.
dmsetup message my_cache 0 invalidate_cblocks 2345 3456-4567 5678-6789
Examples
========
The test suite can be found here:
https://github.com/jthornber/device-mapper-test-suite
dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
/dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0'
dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
/dev/mapper/ssd /dev/mapper/origin 1024 1 writeback \
mq 4 sequential_threshold 1024 random_threshold 8'

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dm-delay
========
Device-Mapper's "delay" target delays reads and/or writes
and maps them to different devices.
Parameters:
<device> <offset> <delay> [<write_device> <write_offset> <write_delay>]
With separate write parameters, the first set is only used for reads.
Delays are specified in milliseconds.
Example scripts
===============
[[
#!/bin/sh
# Create device delaying rw operation for 500ms
echo "0 `blockdev --getsize $1` delay $1 0 500" | dmsetup create delayed
]]
[[
#!/bin/sh
# Create device delaying only write operation for 500ms and
# splitting reads and writes to different devices $1 $2
echo "0 `blockdev --getsize $1` delay $1 0 0 $2 0 500" | dmsetup create delayed
]]

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dm-crypt
=========
Device-Mapper's "crypt" target provides transparent encryption of block devices
using the kernel crypto API.
For a more detailed description of supported parameters see:
http://code.google.com/p/cryptsetup/wiki/DMCrypt
Parameters: <cipher> <key> <iv_offset> <device path> \
<offset> [<#opt_params> <opt_params>]
<cipher>
Encryption cipher and an optional IV generation mode.
(In format cipher[:keycount]-chainmode-ivmode[:ivopts]).
Examples:
des
aes-cbc-essiv:sha256
twofish-ecb
/proc/crypto contains supported crypto modes
<key>
Key used for encryption. It is encoded as a hexadecimal number.
You can only use key sizes that are valid for the selected cipher
in combination with the selected iv mode.
Note that for some iv modes the key string can contain additional
keys (for example IV seed) so the key contains more parts concatenated
into a single string.
<keycount>
Multi-key compatibility mode. You can define <keycount> keys and
then sectors are encrypted according to their offsets (sector 0 uses key0;
sector 1 uses key1 etc.). <keycount> must be a power of two.
<iv_offset>
The IV offset is a sector count that is added to the sector number
before creating the IV.
<device path>
This is the device that is going to be used as backend and contains the
encrypted data. You can specify it as a path like /dev/xxx or a device
number <major>:<minor>.
<offset>
Starting sector within the device where the encrypted data begins.
<#opt_params>
Number of optional parameters. If there are no optional parameters,
the optional paramaters section can be skipped or #opt_params can be zero.
Otherwise #opt_params is the number of following arguments.
Example of optional parameters section:
1 allow_discards
allow_discards
Block discard requests (a.k.a. TRIM) are passed through the crypt device.
The default is to ignore discard requests.
WARNING: Assess the specific security risks carefully before enabling this
option. For example, allowing discards on encrypted devices may lead to
the leak of information about the ciphertext device (filesystem type,
used space etc.) if the discarded blocks can be located easily on the
device later.
Example scripts
===============
LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
encryption with dm-crypt using the 'cryptsetup' utility, see
http://code.google.com/p/cryptsetup/
[[
#!/bin/sh
# Create a crypt device using dmsetup
dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
]]
[[
#!/bin/sh
# Create a crypt device using cryptsetup and LUKS header with default cipher
cryptsetup luksFormat $1
cryptsetup luksOpen $1 crypt1
]]

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dm-flakey
=========
This target is the same as the linear target except that it exhibits
unreliable behaviour periodically. It's been found useful in simulating
failing devices for testing purposes.
Starting from the time the table is loaded, the device is available for
<up interval> seconds, then exhibits unreliable behaviour for <down
interval> seconds, and then this cycle repeats.
Also, consider using this in combination with the dm-delay target too,
which can delay reads and writes and/or send them to different
underlying devices.
Table parameters
----------------
<dev path> <offset> <up interval> <down interval> \
[<num_features> [<feature arguments>]]
Mandatory parameters:
<dev path>: Full pathname to the underlying block-device, or a
"major:minor" device-number.
<offset>: Starting sector within the device.
<up interval>: Number of seconds device is available.
<down interval>: Number of seconds device returns errors.
Optional feature parameters:
If no feature parameters are present, during the periods of
unreliability, all I/O returns errors.
drop_writes:
All write I/O is silently ignored.
Read I/O is handled correctly.
corrupt_bio_byte <Nth_byte> <direction> <value> <flags>:
During <down interval>, replace <Nth_byte> of the data of
each matching bio with <value>.
<Nth_byte>: The offset of the byte to replace.
Counting starts at 1, to replace the first byte.
<direction>: Either 'r' to corrupt reads or 'w' to corrupt writes.
'w' is incompatible with drop_writes.
<value>: The value (from 0-255) to write.
<flags>: Perform the replacement only if bio->bi_rw has all the
selected flags set.
Examples:
corrupt_bio_byte 32 r 1 0
- replaces the 32nd byte of READ bios with the value 1
corrupt_bio_byte 224 w 0 32
- replaces the 224th byte of REQ_META (=32) bios with the value 0

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dm-io
=====
Dm-io provides synchronous and asynchronous I/O services. There are three
types of I/O services available, and each type has a sync and an async
version.
The user must set up an io_region structure to describe the desired location
of the I/O. Each io_region indicates a block-device along with the starting
sector and size of the region.
struct io_region {
struct block_device *bdev;
sector_t sector;
sector_t count;
};
Dm-io can read from one io_region or write to one or more io_regions. Writes
to multiple regions are specified by an array of io_region structures.
The first I/O service type takes a list of memory pages as the data buffer for
the I/O, along with an offset into the first page.
struct page_list {
struct page_list *next;
struct page *page;
};
int dm_io_sync(unsigned int num_regions, struct io_region *where, int rw,
struct page_list *pl, unsigned int offset,
unsigned long *error_bits);
int dm_io_async(unsigned int num_regions, struct io_region *where, int rw,
struct page_list *pl, unsigned int offset,
io_notify_fn fn, void *context);
The second I/O service type takes an array of bio vectors as the data buffer
for the I/O. This service can be handy if the caller has a pre-assembled bio,
but wants to direct different portions of the bio to different devices.
int dm_io_sync_bvec(unsigned int num_regions, struct io_region *where,
int rw, struct bio_vec *bvec,
unsigned long *error_bits);
int dm_io_async_bvec(unsigned int num_regions, struct io_region *where,
int rw, struct bio_vec *bvec,
io_notify_fn fn, void *context);
The third I/O service type takes a pointer to a vmalloc'd memory buffer as the
data buffer for the I/O. This service can be handy if the caller needs to do
I/O to a large region but doesn't want to allocate a large number of individual
memory pages.
int dm_io_sync_vm(unsigned int num_regions, struct io_region *where, int rw,
void *data, unsigned long *error_bits);
int dm_io_async_vm(unsigned int num_regions, struct io_region *where, int rw,
void *data, io_notify_fn fn, void *context);
Callers of the asynchronous I/O services must include the name of a completion
callback routine and a pointer to some context data for the I/O.
typedef void (*io_notify_fn)(unsigned long error, void *context);
The "error" parameter in this callback, as well as the "*error" parameter in
all of the synchronous versions, is a bitset (instead of a simple error value).
In the case of an write-I/O to multiple regions, this bitset allows dm-io to
indicate success or failure on each individual region.
Before using any of the dm-io services, the user should call dm_io_get()
and specify the number of pages they expect to perform I/O on concurrently.
Dm-io will attempt to resize its mempool to make sure enough pages are
always available in order to avoid unnecessary waiting while performing I/O.
When the user is finished using the dm-io services, they should call
dm_io_put() and specify the same number of pages that were given on the
dm_io_get() call.

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Device-Mapper Logging
=====================
The device-mapper logging code is used by some of the device-mapper
RAID targets to track regions of the disk that are not consistent.
A region (or portion of the address space) of the disk may be
inconsistent because a RAID stripe is currently being operated on or
a machine died while the region was being altered. In the case of
mirrors, a region would be considered dirty/inconsistent while you
are writing to it because the writes need to be replicated for all
the legs of the mirror and may not reach the legs at the same time.
Once all writes are complete, the region is considered clean again.
There is a generic logging interface that the device-mapper RAID
implementations use to perform logging operations (see
dm_dirty_log_type in include/linux/dm-dirty-log.h). Various different
logging implementations are available and provide different
capabilities. The list includes:
Type Files
==== =====
disk drivers/md/dm-log.c
core drivers/md/dm-log.c
userspace drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h
The "disk" log type
-------------------
This log implementation commits the log state to disk. This way, the
logging state survives reboots/crashes.
The "core" log type
-------------------
This log implementation keeps the log state in memory. The log state
will not survive a reboot or crash, but there may be a small boost in
performance. This method can also be used if no storage device is
available for storing log state.
The "userspace" log type
------------------------
This log type simply provides a way to export the log API to userspace,
so log implementations can be done there. This is done by forwarding most
logging requests to userspace, where a daemon receives and processes the
request.
The structure used for communication between kernel and userspace are
located in include/linux/dm-log-userspace.h. Due to the frequency,
diversity, and 2-way communication nature of the exchanges between
kernel and userspace, 'connector' is used as the interface for
communication.
There are currently two userspace log implementations that leverage this
framework - "clustered-disk" and "clustered-core". These implementations
provide a cluster-coherent log for shared-storage. Device-mapper mirroring
can be used in a shared-storage environment when the cluster log implementations
are employed.

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dm-queue-length
===============
dm-queue-length is a path selector module for device-mapper targets,
which selects a path with the least number of in-flight I/Os.
The path selector name is 'queue-length'.
Table parameters for each path: [<repeat_count>]
<repeat_count>: The number of I/Os to dispatch using the selected
path before switching to the next path.
If not given, internal default is used. To check
the default value, see the activated table.
Status for each path: <status> <fail-count> <in-flight>
<status>: 'A' if the path is active, 'F' if the path is failed.
<fail-count>: The number of path failures.
<in-flight>: The number of in-flight I/Os on the path.
Algorithm
=========
dm-queue-length increments/decrements 'in-flight' when an I/O is
dispatched/completed respectively.
dm-queue-length selects a path with the minimum 'in-flight'.
Examples
========
In case that 2 paths (sda and sdb) are used with repeat_count == 128.
# echo "0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128" \
dmsetup create test
#
# dmsetup table
test: 0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128
#
# dmsetup status
test: 0 10 multipath 2 0 0 0 1 1 E 0 2 1 8:0 A 0 0 8:16 A 0 0

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dm-raid
=======
The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.
It allows the MD RAID drivers to be accessed using a device-mapper
interface.
Mapping Table Interface
-----------------------
The target is named "raid" and it accepts the following parameters:
<raid_type> <#raid_params> <raid_params> \
<#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]
<raid_type>:
raid1 RAID1 mirroring
raid4 RAID4 dedicated parity disk
raid5_la RAID5 left asymmetric
- rotating parity 0 with data continuation
raid5_ra RAID5 right asymmetric
- rotating parity N with data continuation
raid5_ls RAID5 left symmetric
- rotating parity 0 with data restart
raid5_rs RAID5 right symmetric
- rotating parity N with data restart
raid6_zr RAID6 zero restart
- rotating parity zero (left-to-right) with data restart
raid6_nr RAID6 N restart
- rotating parity N (right-to-left) with data restart
raid6_nc RAID6 N continue
- rotating parity N (right-to-left) with data continuation
raid10 Various RAID10 inspired algorithms chosen by additional params
- RAID10: Striped Mirrors (aka 'Striping on top of mirrors')
- RAID1E: Integrated Adjacent Stripe Mirroring
- RAID1E: Integrated Offset Stripe Mirroring
- and other similar RAID10 variants
Reference: Chapter 4 of
http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
<#raid_params>: The number of parameters that follow.
<raid_params> consists of
Mandatory parameters:
<chunk_size>: Chunk size in sectors. This parameter is often known as
"stripe size". It is the only mandatory parameter and
is placed first.
followed by optional parameters (in any order):
[sync|nosync] Force or prevent RAID initialization.
[rebuild <idx>] Rebuild drive number 'idx' (first drive is 0).
[daemon_sleep <ms>]
Interval between runs of the bitmap daemon that
clear bits. A longer interval means less bitmap I/O but
resyncing after a failure is likely to take longer.
[min_recovery_rate <kB/sec/disk>] Throttle RAID initialization
[max_recovery_rate <kB/sec/disk>] Throttle RAID initialization
[write_mostly <idx>] Mark drive index 'idx' write-mostly.
[max_write_behind <sectors>] See '--write-behind=' (man mdadm)
[stripe_cache <sectors>] Stripe cache size (RAID 4/5/6 only)
[region_size <sectors>]
The region_size multiplied by the number of regions is the
logical size of the array. The bitmap records the device
synchronisation state for each region.
[raid10_copies <# copies>]
[raid10_format <near|far|offset>]
These two options are used to alter the default layout of
a RAID10 configuration. The number of copies is can be
specified, but the default is 2. There are also three
variations to how the copies are laid down - the default
is "near". Near copies are what most people think of with
respect to mirroring. If these options are left unspecified,
or 'raid10_copies 2' and/or 'raid10_format near' are given,
then the layouts for 2, 3 and 4 devices are:
2 drives 3 drives 4 drives
-------- ---------- --------------
A1 A1 A1 A1 A2 A1 A1 A2 A2
A2 A2 A2 A3 A3 A3 A3 A4 A4
A3 A3 A4 A4 A5 A5 A5 A6 A6
A4 A4 A5 A6 A6 A7 A7 A8 A8
.. .. .. .. .. .. .. .. ..
The 2-device layout is equivalent 2-way RAID1. The 4-device
layout is what a traditional RAID10 would look like. The
3-device layout is what might be called a 'RAID1E - Integrated
Adjacent Stripe Mirroring'.
If 'raid10_copies 2' and 'raid10_format far', then the layouts
for 2, 3 and 4 devices are:
2 drives 3 drives 4 drives
-------- -------------- --------------------
A1 A2 A1 A2 A3 A1 A2 A3 A4
A3 A4 A4 A5 A6 A5 A6 A7 A8
A5 A6 A7 A8 A9 A9 A10 A11 A12
.. .. .. .. .. .. .. .. ..
A2 A1 A3 A1 A2 A2 A1 A4 A3
A4 A3 A6 A4 A5 A6 A5 A8 A7
A6 A5 A9 A7 A8 A10 A9 A12 A11
.. .. .. .. .. .. .. .. ..
If 'raid10_copies 2' and 'raid10_format offset', then the
layouts for 2, 3 and 4 devices are:
2 drives 3 drives 4 drives
-------- ------------ -----------------
A1 A2 A1 A2 A3 A1 A2 A3 A4
A2 A1 A3 A1 A2 A2 A1 A4 A3
A3 A4 A4 A5 A6 A5 A6 A7 A8
A4 A3 A6 A4 A5 A6 A5 A8 A7
A5 A6 A7 A8 A9 A9 A10 A11 A12
A6 A5 A9 A7 A8 A10 A9 A12 A11
.. .. .. .. .. .. .. .. ..
Here we see layouts closely akin to 'RAID1E - Integrated
Offset Stripe Mirroring'.
<#raid_devs>: The number of devices composing the array.
Each device consists of two entries. The first is the device
containing the metadata (if any); the second is the one containing the
data.
If a drive has failed or is missing at creation time, a '-' can be
given for both the metadata and data drives for a given position.
Example Tables
--------------
# RAID4 - 4 data drives, 1 parity (no metadata devices)
# No metadata devices specified to hold superblock/bitmap info
# Chunk size of 1MiB
# (Lines separated for easy reading)
0 1960893648 raid \
raid4 1 2048 \
5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81
# RAID4 - 4 data drives, 1 parity (with metadata devices)
# Chunk size of 1MiB, force RAID initialization,
# min recovery rate at 20 kiB/sec/disk
0 1960893648 raid \
raid4 4 2048 sync min_recovery_rate 20 \
5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82
Status Output
-------------
'dmsetup table' displays the table used to construct the mapping.
The optional parameters are always printed in the order listed
above with "sync" or "nosync" always output ahead of the other
arguments, regardless of the order used when originally loading the table.
Arguments that can be repeated are ordered by value.
'dmsetup status' yields information on the state and health of the array.
The output is as follows (normally a single line, but expanded here for
clarity):
1: <s> <l> raid \
2: <raid_type> <#devices> <health_chars> \
3: <sync_ratio> <sync_action> <mismatch_cnt>
Line 1 is the standard output produced by device-mapper.
Line 2 & 3 are produced by the raid target and are best explained by example:
0 1960893648 raid raid4 5 AAAAA 2/490221568 init 0
Here we can see the RAID type is raid4, there are 5 devices - all of
which are 'A'live, and the array is 2/490221568 complete with its initial
recovery. Here is a fuller description of the individual fields:
<raid_type> Same as the <raid_type> used to create the array.
<health_chars> One char for each device, indicating: 'A' = alive and
in-sync, 'a' = alive but not in-sync, 'D' = dead/failed.
<sync_ratio> The ratio indicating how much of the array has undergone
the process described by 'sync_action'. If the
'sync_action' is "check" or "repair", then the process
of "resync" or "recover" can be considered complete.
<sync_action> One of the following possible states:
idle - No synchronization action is being performed.
frozen - The current action has been halted.
resync - Array is undergoing its initial synchronization
or is resynchronizing after an unclean shutdown
(possibly aided by a bitmap).
recover - A device in the array is being rebuilt or
replaced.
check - A user-initiated full check of the array is
being performed. All blocks are read and
checked for consistency. The number of
discrepancies found are recorded in
<mismatch_cnt>. No changes are made to the
array by this action.
repair - The same as "check", but discrepancies are
corrected.
reshape - The array is undergoing a reshape.
<mismatch_cnt> The number of discrepancies found between mirror copies
in RAID1/10 or wrong parity values found in RAID4/5/6.
This value is valid only after a "check" of the array
is performed. A healthy array has a 'mismatch_cnt' of 0.
Message Interface
-----------------
The dm-raid target will accept certain actions through the 'message' interface.
('man dmsetup' for more information on the message interface.) These actions
include:
"idle" - Halt the current sync action.
"frozen" - Freeze the current sync action.
"resync" - Initiate/continue a resync.
"recover"- Initiate/continue a recover process.
"check" - Initiate a check (i.e. a "scrub") of the array.
"repair" - Initiate a repair of the array.
"reshape"- Currently unsupported (-EINVAL).
Version History
---------------
1.0.0 Initial version. Support for RAID 4/5/6
1.1.0 Added support for RAID 1
1.2.0 Handle creation of arrays that contain failed devices.
1.3.0 Added support for RAID 10
1.3.1 Allow device replacement/rebuild for RAID 10
1.3.2 Fix/improve redundancy checking for RAID10
1.4.0 Non-functional change. Removes arg from mapping function.
1.4.1 RAID10 fix redundancy validation checks (commit 55ebbb5).
1.4.2 Add RAID10 "far" and "offset" algorithm support.
1.5.0 Add message interface to allow manipulation of the sync_action.
New status (STATUSTYPE_INFO) fields: sync_action and mismatch_cnt.
1.5.1 Add ability to restore transiently failed devices on resume.
1.5.2 'mismatch_cnt' is zero unless [last_]sync_action is "check".

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dm-service-time
===============
dm-service-time is a path selector module for device-mapper targets,
which selects a path with the shortest estimated service time for
the incoming I/O.
The service time for each path is estimated by dividing the total size
of in-flight I/Os on a path with the performance value of the path.
The performance value is a relative throughput value among all paths
in a path-group, and it can be specified as a table argument.
The path selector name is 'service-time'.
Table parameters for each path: [<repeat_count> [<relative_throughput>]]
<repeat_count>: The number of I/Os to dispatch using the selected
path before switching to the next path.
If not given, internal default is used. To check
the default value, see the activated table.
<relative_throughput>: The relative throughput value of the path
among all paths in the path-group.
The valid range is 0-100.
If not given, minimum value '1' is used.
If '0' is given, the path isn't selected while
other paths having a positive value are available.
Status for each path: <status> <fail-count> <in-flight-size> \
<relative_throughput>
<status>: 'A' if the path is active, 'F' if the path is failed.
<fail-count>: The number of path failures.
<in-flight-size>: The size of in-flight I/Os on the path.
<relative_throughput>: The relative throughput value of the path
among all paths in the path-group.
Algorithm
=========
dm-service-time adds the I/O size to 'in-flight-size' when the I/O is
dispatched and subtracts when completed.
Basically, dm-service-time selects a path having minimum service time
which is calculated by:
('in-flight-size' + 'size-of-incoming-io') / 'relative_throughput'
However, some optimizations below are used to reduce the calculation
as much as possible.
1. If the paths have the same 'relative_throughput', skip
the division and just compare the 'in-flight-size'.
2. If the paths have the same 'in-flight-size', skip the division
and just compare the 'relative_throughput'.
3. If some paths have non-zero 'relative_throughput' and others
have zero 'relative_throughput', ignore those paths with zero
'relative_throughput'.
If such optimizations can't be applied, calculate service time, and
compare service time.
If calculated service time is equal, the path having maximum
'relative_throughput' may be better. So compare 'relative_throughput'
then.
Examples
========
In case that 2 paths (sda and sdb) are used with repeat_count == 128
and sda has an average throughput 1GB/s and sdb has 4GB/s,
'relative_throughput' value may be '1' for sda and '4' for sdb.
# echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4" \
dmsetup create test
#
# dmsetup table
test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4
#
# dmsetup status
test: 0 10 multipath 2 0 0 0 1 1 E 0 2 2 8:0 A 0 0 1 8:16 A 0 0 4
Or '2' for sda and '8' for sdb would be also true.
# echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8" \
dmsetup create test
#
# dmsetup table
test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8
#
# dmsetup status
test: 0 10 multipath 2 0 0 0 1 1 E 0 2 2 8:0 A 0 0 2 8:16 A 0 0 8

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The device-mapper uevent code adds the capability to device-mapper to create
and send kobject uevents (uevents). Previously device-mapper events were only
available through the ioctl interface. The advantage of the uevents interface
is the event contains environment attributes providing increased context for
the event avoiding the need to query the state of the device-mapper device after
the event is received.
There are two functions currently for device-mapper events. The first function
listed creates the event and the second function sends the event(s).
void dm_path_uevent(enum dm_uevent_type event_type, struct dm_target *ti,
const char *path, unsigned nr_valid_paths)
void dm_send_uevents(struct list_head *events, struct kobject *kobj)
The variables added to the uevent environment are:
Variable Name: DM_TARGET
Uevent Action(s): KOBJ_CHANGE
Type: string
Description:
Value: Name of device-mapper target that generated the event.
Variable Name: DM_ACTION
Uevent Action(s): KOBJ_CHANGE
Type: string
Description:
Value: Device-mapper specific action that caused the uevent action.
PATH_FAILED - A path has failed.
PATH_REINSTATED - A path has been reinstated.
Variable Name: DM_SEQNUM
Uevent Action(s): KOBJ_CHANGE
Type: unsigned integer
Description: A sequence number for this specific device-mapper device.
Value: Valid unsigned integer range.
Variable Name: DM_PATH
Uevent Action(s): KOBJ_CHANGE
Type: string
Description: Major and minor number of the path device pertaining to this
event.
Value: Path name in the form of "Major:Minor"
Variable Name: DM_NR_VALID_PATHS
Uevent Action(s): KOBJ_CHANGE
Type: unsigned integer
Description:
Value: Valid unsigned integer range.
Variable Name: DM_NAME
Uevent Action(s): KOBJ_CHANGE
Type: string
Description: Name of the device-mapper device.
Value: Name
Variable Name: DM_UUID
Uevent Action(s): KOBJ_CHANGE
Type: string
Description: UUID of the device-mapper device.
Value: UUID. (Empty string if there isn't one.)
An example of the uevents generated as captured by udevmonitor is shown
below.
1.) Path failure.
UEVENT[1192521009.711215] change@/block/dm-3
ACTION=change
DEVPATH=/block/dm-3
SUBSYSTEM=block
DM_TARGET=multipath
DM_ACTION=PATH_FAILED
DM_SEQNUM=1
DM_PATH=8:32
DM_NR_VALID_PATHS=0
DM_NAME=mpath2
DM_UUID=mpath-35333333000002328
MINOR=3
MAJOR=253
SEQNUM=1130
2.) Path reinstate.
UEVENT[1192521132.989927] change@/block/dm-3
ACTION=change
DEVPATH=/block/dm-3
SUBSYSTEM=block
DM_TARGET=multipath
DM_ACTION=PATH_REINSTATED
DM_SEQNUM=2
DM_PATH=8:32
DM_NR_VALID_PATHS=1
DM_NAME=mpath2
DM_UUID=mpath-35333333000002328
MINOR=3
MAJOR=253
SEQNUM=1131

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Introduction
============
dm-era is a target that behaves similar to the linear target. In
addition it keeps track of which blocks were written within a user
defined period of time called an 'era'. Each era target instance
maintains the current era as a monotonically increasing 32-bit
counter.
Use cases include tracking changed blocks for backup software, and
partially invalidating the contents of a cache to restore cache
coherency after rolling back a vendor snapshot.
Constructor
===========
era <metadata dev> <origin dev> <block size>
metadata dev : fast device holding the persistent metadata
origin dev : device holding data blocks that may change
block size : block size of origin data device, granularity that is
tracked by the target
Messages
========
None of the dm messages take any arguments.
checkpoint
----------
Possibly move to a new era. You shouldn't assume the era has
incremented. After sending this message, you should check the
current era via the status line.
take_metadata_snap
------------------
Create a clone of the metadata, to allow a userland process to read it.
drop_metadata_snap
------------------
Drop the metadata snapshot.
Status
======
<metadata block size> <#used metadata blocks>/<#total metadata blocks>
<current era> <held metadata root | '-'>
metadata block size : Fixed block size for each metadata block in
sectors
#used metadata blocks : Number of metadata blocks used
#total metadata blocks : Total number of metadata blocks
current era : The current era
held metadata root : The location, in blocks, of the metadata root
that has been 'held' for userspace read
access. '-' indicates there is no held root
Detailed use case
=================
The scenario of invalidating a cache when rolling back a vendor
snapshot was the primary use case when developing this target:
Taking a vendor snapshot
------------------------
- Send a checkpoint message to the era target
- Make a note of the current era in its status line
- Take vendor snapshot (the era and snapshot should be forever
associated now).
Rolling back to an vendor snapshot
----------------------------------
- Cache enters passthrough mode (see: dm-cache's docs in cache.txt)
- Rollback vendor storage
- Take metadata snapshot
- Ascertain which blocks have been written since the snapshot was taken
by checking each block's era
- Invalidate those blocks in the caching software
- Cache returns to writeback/writethrough mode
Memory usage
============
The target uses a bitset to record writes in the current era. It also
has a spare bitset ready for switching over to a new era. Other than
that it uses a few 4k blocks for updating metadata.
(4 * nr_blocks) bytes + buffers
Resilience
==========
Metadata is updated on disk before a write to a previously unwritten
block is performed. As such dm-era should not be effected by a hard
crash such as power failure.
Userland tools
==============
Userland tools are found in the increasingly poorly named
thin-provisioning-tools project:
https://github.com/jthornber/thin-provisioning-tools

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kcopyd
======
Kcopyd provides the ability to copy a range of sectors from one block-device
to one or more other block-devices, with an asynchronous completion
notification. It is used by dm-snapshot and dm-mirror.
Users of kcopyd must first create a client and indicate how many memory pages
to set aside for their copy jobs. This is done with a call to
kcopyd_client_create().
int kcopyd_client_create(unsigned int num_pages,
struct kcopyd_client **result);
To start a copy job, the user must set up io_region structures to describe
the source and destinations of the copy. Each io_region indicates a
block-device along with the starting sector and size of the region. The source
of the copy is given as one io_region structure, and the destinations of the
copy are given as an array of io_region structures.
struct io_region {
struct block_device *bdev;
sector_t sector;
sector_t count;
};
To start the copy, the user calls kcopyd_copy(), passing in the client
pointer, pointers to the source and destination io_regions, the name of a
completion callback routine, and a pointer to some context data for the copy.
int kcopyd_copy(struct kcopyd_client *kc, struct io_region *from,
unsigned int num_dests, struct io_region *dests,
unsigned int flags, kcopyd_notify_fn fn, void *context);
typedef void (*kcopyd_notify_fn)(int read_err, unsigned int write_err,
void *context);
When the copy completes, kcopyd will call the user's completion routine,
passing back the user's context pointer. It will also indicate if a read or
write error occurred during the copy.
When a user is done with all their copy jobs, they should call
kcopyd_client_destroy() to delete the kcopyd client, which will release the
associated memory pages.
void kcopyd_client_destroy(struct kcopyd_client *kc);

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dm-linear
=========
Device-Mapper's "linear" target maps a linear range of the Device-Mapper
device onto a linear range of another device. This is the basic building
block of logical volume managers.
Parameters: <dev path> <offset>
<dev path>: Full pathname to the underlying block-device, or a
"major:minor" device-number.
<offset>: Starting sector within the device.
Example scripts
===============
[[
#!/bin/sh
# Create an identity mapping for a device
echo "0 `blockdev --getsize $1` linear $1 0" | dmsetup create identity
]]
[[
#!/bin/sh
# Join 2 devices together
size1=`blockdev --getsize $1`
size2=`blockdev --getsize $2`
echo "0 $size1 linear $1 0
$size1 $size2 linear $2 0" | dmsetup create joined
]]
[[
#!/usr/bin/perl -w
# Split a device into 4M chunks and then join them together in reverse order.
my $name = "reverse";
my $extent_size = 4 * 1024 * 2;
my $dev = $ARGV[0];
my $table = "";
my $count = 0;
if (!defined($dev)) {
die("Please specify a device.\n");
}
my $dev_size = `blockdev --getsize $dev`;
my $extents = int($dev_size / $extent_size) -
(($dev_size % $extent_size) ? 1 : 0);
while ($extents > 0) {
my $this_start = $count * $extent_size;
$extents--;
$count++;
my $this_offset = $extents * $extent_size;
$table .= "$this_start $extent_size linear $dev $this_offset\n";
}
`echo \"$table\" | dmsetup create $name`;
]]

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Introduction
============
The more-sophisticated device-mapper targets require complex metadata
that is managed in kernel. In late 2010 we were seeing that various
different targets were rolling their own data structures, for example:
- Mikulas Patocka's multisnap implementation
- Heinz Mauelshagen's thin provisioning target
- Another btree-based caching target posted to dm-devel
- Another multi-snapshot target based on a design of Daniel Phillips
Maintaining these data structures takes a lot of work, so if possible
we'd like to reduce the number.
The persistent-data library is an attempt to provide a re-usable
framework for people who want to store metadata in device-mapper
targets. It's currently used by the thin-provisioning target and an
upcoming hierarchical storage target.
Overview
========
The main documentation is in the header files which can all be found
under drivers/md/persistent-data.
The block manager
-----------------
dm-block-manager.[hc]
This provides access to the data on disk in fixed sized-blocks. There
is a read/write locking interface to prevent concurrent accesses, and
keep data that is being used in the cache.
Clients of persistent-data are unlikely to use this directly.
The transaction manager
-----------------------
dm-transaction-manager.[hc]
This restricts access to blocks and enforces copy-on-write semantics.
The only way you can get hold of a writable block through the
transaction manager is by shadowing an existing block (ie. doing
copy-on-write) or allocating a fresh one. Shadowing is elided within
the same transaction so performance is reasonable. The commit method
ensures that all data is flushed before it writes the superblock.
On power failure your metadata will be as it was when last committed.
The Space Maps
--------------
dm-space-map.h
dm-space-map-metadata.[hc]
dm-space-map-disk.[hc]
On-disk data structures that keep track of reference counts of blocks.
Also acts as the allocator of new blocks. Currently two
implementations: a simpler one for managing blocks on a different
device (eg. thinly-provisioned data blocks); and one for managing
the metadata space. The latter is complicated by the need to store
its own data within the space it's managing.
The data structures
-------------------
dm-btree.[hc]
dm-btree-remove.c
dm-btree-spine.c
dm-btree-internal.h
Currently there is only one data structure, a hierarchical btree.
There are plans to add more. For example, something with an
array-like interface would see a lot of use.
The btree is 'hierarchical' in that you can define it to be composed
of nested btrees, and take multiple keys. For example, the
thin-provisioning target uses a btree with two levels of nesting.
The first maps a device id to a mapping tree, and that in turn maps a
virtual block to a physical block.
Values stored in the btrees can have arbitrary size. Keys are always
64bits, although nesting allows you to use multiple keys.

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Device-mapper snapshot support
==============================
Device-mapper allows you, without massive data copying:
*) To create snapshots of any block device i.e. mountable, saved states of
the block device which are also writable without interfering with the
original content;
*) To create device "forks", i.e. multiple different versions of the
same data stream.
*) To merge a snapshot of a block device back into the snapshot's origin
device.
In the first two cases, dm copies only the chunks of data that get
changed and uses a separate copy-on-write (COW) block device for
storage.
For snapshot merge the contents of the COW storage are merged back into
the origin device.
There are three dm targets available:
snapshot, snapshot-origin, and snapshot-merge.
*) snapshot-origin <origin>
which will normally have one or more snapshots based on it.
Reads will be mapped directly to the backing device. For each write, the
original data will be saved in the <COW device> of each snapshot to keep
its visible content unchanged, at least until the <COW device> fills up.
*) snapshot <origin> <COW device> <persistent?> <chunksize>
A snapshot of the <origin> block device is created. Changed chunks of
<chunksize> sectors will be stored on the <COW device>. Writes will
only go to the <COW device>. Reads will come from the <COW device> or
from <origin> for unchanged data. <COW device> will often be
smaller than the origin and if it fills up the snapshot will become
useless and be disabled, returning errors. So it is important to monitor
the amount of free space and expand the <COW device> before it fills up.
<persistent?> is P (Persistent) or N (Not persistent - will not survive
after reboot).
The difference is that for transient snapshots less metadata must be
saved on disk - they can be kept in memory by the kernel.
* snapshot-merge <origin> <COW device> <persistent> <chunksize>
takes the same table arguments as the snapshot target except it only
works with persistent snapshots. This target assumes the role of the
"snapshot-origin" target and must not be loaded if the "snapshot-origin"
is still present for <origin>.
Creates a merging snapshot that takes control of the changed chunks
stored in the <COW device> of an existing snapshot, through a handover
procedure, and merges these chunks back into the <origin>. Once merging
has started (in the background) the <origin> may be opened and the merge
will continue while I/O is flowing to it. Changes to the <origin> are
deferred until the merging snapshot's corresponding chunk(s) have been
merged. Once merging has started the snapshot device, associated with
the "snapshot" target, will return -EIO when accessed.
How snapshot is used by LVM2
============================
When you create the first LVM2 snapshot of a volume, four dm devices are used:
1) a device containing the original mapping table of the source volume;
2) a device used as the <COW device>;
3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
volume;
4) the "original" volume (which uses the device number used by the original
source volume), whose table is replaced by a "snapshot-origin" mapping
from device #1.
A fixed naming scheme is used, so with the following commands:
lvcreate -L 1G -n base volumeGroup
lvcreate -L 100M --snapshot -n snap volumeGroup/base
we'll have this situation (with volumes in above order):
# dmsetup table|grep volumeGroup
volumeGroup-base-real: 0 2097152 linear 8:19 384
volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
volumeGroup-base: 0 2097152 snapshot-origin 254:11
# ls -lL /dev/mapper/volumeGroup-*
brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
brw------- 1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
brw------- 1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
brw------- 1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
How snapshot-merge is used by LVM2
==================================
A merging snapshot assumes the role of the "snapshot-origin" while
merging. As such the "snapshot-origin" is replaced with
"snapshot-merge". The "-real" device is not changed and the "-cow"
device is renamed to <origin name>-cow to aid LVM2's cleanup of the
merging snapshot after it completes. The "snapshot" that hands over its
COW device to the "snapshot-merge" is deactivated (unless using lvchange
--refresh); but if it is left active it will simply return I/O errors.
A snapshot will merge into its origin with the following command:
lvconvert --merge volumeGroup/snap
we'll now have this situation:
# dmsetup table|grep volumeGroup
volumeGroup-base-real: 0 2097152 linear 8:19 384
volumeGroup-base-cow: 0 204800 linear 8:19 2097536
volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
# ls -lL /dev/mapper/volumeGroup-*
brw------- 1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
brw------- 1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
brw------- 1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
How to determine when a merging is complete
===========================================
The snapshot-merge and snapshot status lines end with:
<sectors_allocated>/<total_sectors> <metadata_sectors>
Both <sectors_allocated> and <total_sectors> include both data and metadata.
During merging, the number of sectors allocated gets smaller and
smaller. Merging has finished when the number of sectors holding data
is zero, in other words <sectors_allocated> == <metadata_sectors>.
Here is a practical example (using a hybrid of lvm and dmsetup commands):
# lvs
LV VG Attr LSize Origin Snap% Move Log Copy% Convert
base volumeGroup owi-a- 4.00g
snap volumeGroup swi-a- 1.00g base 18.97
# dmsetup status volumeGroup-snap
0 8388608 snapshot 397896/2097152 1560
^^^^ metadata sectors
# lvconvert --merge -b volumeGroup/snap
Merging of volume snap started.
# lvs volumeGroup/snap
LV VG Attr LSize Origin Snap% Move Log Copy% Convert
base volumeGroup Owi-a- 4.00g 17.23
# dmsetup status volumeGroup-base
0 8388608 snapshot-merge 281688/2097152 1104
# dmsetup status volumeGroup-base
0 8388608 snapshot-merge 180480/2097152 712
# dmsetup status volumeGroup-base
0 8388608 snapshot-merge 16/2097152 16
Merging has finished.
# lvs
LV VG Attr LSize Origin Snap% Move Log Copy% Convert
base volumeGroup owi-a- 4.00g

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DM statistics
=============
Device Mapper supports the collection of I/O statistics on user-defined
regions of a DM device. If no regions are defined no statistics are
collected so there isn't any performance impact. Only bio-based DM
devices are currently supported.
Each user-defined region specifies a starting sector, length and step.
Individual statistics will be collected for each step-sized area within
the range specified.
The I/O statistics counters for each step-sized area of a region are
in the same format as /sys/block/*/stat or /proc/diskstats (see:
Documentation/iostats.txt). But two extra counters (12 and 13) are
provided: total time spent reading and writing in milliseconds. All
these counters may be accessed by sending the @stats_print message to
the appropriate DM device via dmsetup.
Each region has a corresponding unique identifier, which we call a
region_id, that is assigned when the region is created. The region_id
must be supplied when querying statistics about the region, deleting the
region, etc. Unique region_ids enable multiple userspace programs to
request and process statistics for the same DM device without stepping
on each other's data.
The creation of DM statistics will allocate memory via kmalloc or
fallback to using vmalloc space. At most, 1/4 of the overall system
memory may be allocated by DM statistics. The admin can see how much
memory is used by reading
/sys/module/dm_mod/parameters/stats_current_allocated_bytes
Messages
========
@stats_create <range> <step> [<program_id> [<aux_data>]]
Create a new region and return the region_id.
<range>
"-" - whole device
"<start_sector>+<length>" - a range of <length> 512-byte sectors
starting with <start_sector>.
<step>
"<area_size>" - the range is subdivided into areas each containing
<area_size> sectors.
"/<number_of_areas>" - the range is subdivided into the specified
number of areas.
<program_id>
An optional parameter. A name that uniquely identifies
the userspace owner of the range. This groups ranges together
so that userspace programs can identify the ranges they
created and ignore those created by others.
The kernel returns this string back in the output of
@stats_list message, but it doesn't use it for anything else.
<aux_data>
An optional parameter. A word that provides auxiliary data
that is useful to the client program that created the range.
The kernel returns this string back in the output of
@stats_list message, but it doesn't use this value for anything.
@stats_delete <region_id>
Delete the region with the specified id.
<region_id>
region_id returned from @stats_create
@stats_clear <region_id>
Clear all the counters except the in-flight i/o counters.
<region_id>
region_id returned from @stats_create
@stats_list [<program_id>]
List all regions registered with @stats_create.
<program_id>
An optional parameter.
If this parameter is specified, only matching regions
are returned.
If it is not specified, all regions are returned.
Output format:
<region_id>: <start_sector>+<length> <step> <program_id> <aux_data>
@stats_print <region_id> [<starting_line> <number_of_lines>]
Print counters for each step-sized area of a region.
<region_id>
region_id returned from @stats_create
<starting_line>
The index of the starting line in the output.
If omitted, all lines are returned.
<number_of_lines>
The number of lines to include in the output.
If omitted, all lines are returned.
Output format for each step-sized area of a region:
<start_sector>+<length> counters
The first 11 counters have the same meaning as
/sys/block/*/stat or /proc/diskstats.
Please refer to Documentation/iostats.txt for details.
1. the number of reads completed
2. the number of reads merged
3. the number of sectors read
4. the number of milliseconds spent reading
5. the number of writes completed
6. the number of writes merged
7. the number of sectors written
8. the number of milliseconds spent writing
9. the number of I/Os currently in progress
10. the number of milliseconds spent doing I/Os
11. the weighted number of milliseconds spent doing I/Os
Additional counters:
12. the total time spent reading in milliseconds
13. the total time spent writing in milliseconds
@stats_print_clear <region_id> [<starting_line> <number_of_lines>]
Atomically print and then clear all the counters except the
in-flight i/o counters. Useful when the client consuming the
statistics does not want to lose any statistics (those updated
between printing and clearing).
<region_id>
region_id returned from @stats_create
<starting_line>
The index of the starting line in the output.
If omitted, all lines are printed and then cleared.
<number_of_lines>
The number of lines to process.
If omitted, all lines are printed and then cleared.
@stats_set_aux <region_id> <aux_data>
Store auxiliary data aux_data for the specified region.
<region_id>
region_id returned from @stats_create
<aux_data>
The string that identifies data which is useful to the client
program that created the range. The kernel returns this
string back in the output of @stats_list message, but it
doesn't use this value for anything.
Examples
========
Subdivide the DM device 'vol' into 100 pieces and start collecting
statistics on them:
dmsetup message vol 0 @stats_create - /100
Set the auxillary data string to "foo bar baz" (the escape for each
space must also be escaped, otherwise the shell will consume them):
dmsetup message vol 0 @stats_set_aux 0 foo\\ bar\\ baz
List the statistics:
dmsetup message vol 0 @stats_list
Print the statistics:
dmsetup message vol 0 @stats_print 0
Delete the statistics:
dmsetup message vol 0 @stats_delete 0

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dm-stripe
=========
Device-Mapper's "striped" target is used to create a striped (i.e. RAID-0)
device across one or more underlying devices. Data is written in "chunks",
with consecutive chunks rotating among the underlying devices. This can
potentially provide improved I/O throughput by utilizing several physical
devices in parallel.
Parameters: <num devs> <chunk size> [<dev path> <offset>]+
<num devs>: Number of underlying devices.
<chunk size>: Size of each chunk of data. Must be at least as
large as the system's PAGE_SIZE.
<dev path>: Full pathname to the underlying block-device, or a
"major:minor" device-number.
<offset>: Starting sector within the device.
One or more underlying devices can be specified. The striped device size must
be a multiple of the chunk size multiplied by the number of underlying devices.
Example scripts
===============
[[
#!/usr/bin/perl -w
# Create a striped device across any number of underlying devices. The device
# will be called "stripe_dev" and have a chunk-size of 128k.
my $chunk_size = 128 * 2;
my $dev_name = "stripe_dev";
my $num_devs = @ARGV;
my @devs = @ARGV;
my ($min_dev_size, $stripe_dev_size, $i);
if (!$num_devs) {
die("Specify at least one device\n");
}
$min_dev_size = `blockdev --getsize $devs[0]`;
for ($i = 1; $i < $num_devs; $i++) {
my $this_size = `blockdev --getsize $devs[$i]`;
$min_dev_size = ($min_dev_size < $this_size) ?
$min_dev_size : $this_size;
}
$stripe_dev_size = $min_dev_size * $num_devs;
$stripe_dev_size -= $stripe_dev_size % ($chunk_size * $num_devs);
$table = "0 $stripe_dev_size striped $num_devs $chunk_size";
for ($i = 0; $i < $num_devs; $i++) {
$table .= " $devs[$i] 0";
}
`echo $table | dmsetup create $dev_name`;
]]

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dm-switch
=========
The device-mapper switch target creates a device that supports an
arbitrary mapping of fixed-size regions of I/O across a fixed set of
paths. The path used for any specific region can be switched
dynamically by sending the target a message.
It maps I/O to underlying block devices efficiently when there is a large
number of fixed-sized address regions but there is no simple pattern
that would allow for a compact representation of the mapping such as
dm-stripe.
Background
----------
Dell EqualLogic and some other iSCSI storage arrays use a distributed
frameless architecture. In this architecture, the storage group
consists of a number of distinct storage arrays ("members") each having
independent controllers, disk storage and network adapters. When a LUN
is created it is spread across multiple members. The details of the
spreading are hidden from initiators connected to this storage system.
The storage group exposes a single target discovery portal, no matter
how many members are being used. When iSCSI sessions are created, each
session is connected to an eth port on a single member. Data to a LUN
can be sent on any iSCSI session, and if the blocks being accessed are
stored on another member the I/O will be forwarded as required. This
forwarding is invisible to the initiator. The storage layout is also
dynamic, and the blocks stored on disk may be moved from member to
member as needed to balance the load.
This architecture simplifies the management and configuration of both
the storage group and initiators. In a multipathing configuration, it
is possible to set up multiple iSCSI sessions to use multiple network
interfaces on both the host and target to take advantage of the
increased network bandwidth. An initiator could use a simple round
robin algorithm to send I/O across all paths and let the storage array
members forward it as necessary, but there is a performance advantage to
sending data directly to the correct member.
A device-mapper table already lets you map different regions of a
device onto different targets. However in this architecture the LUN is
spread with an address region size on the order of 10s of MBs, which
means the resulting table could have more than a million entries and
consume far too much memory.
Using this device-mapper switch target we can now build a two-layer
device hierarchy:
Upper Tier – Determine which array member the I/O should be sent to.
Lower Tier – Load balance amongst paths to a particular member.
The lower tier consists of a single dm multipath device for each member.
Each of these multipath devices contains the set of paths directly to
the array member in one priority group, and leverages existing path
selectors to load balance amongst these paths. We also build a
non-preferred priority group containing paths to other array members for
failover reasons.
The upper tier consists of a single dm-switch device. This device uses
a bitmap to look up the location of the I/O and choose the appropriate
lower tier device to route the I/O. By using a bitmap we are able to
use 4 bits for each address range in a 16 member group (which is very
large for us). This is a much denser representation than the dm table
b-tree can achieve.
Construction Parameters
=======================
<num_paths> <region_size> <num_optional_args> [<optional_args>...]
[<dev_path> <offset>]+
<num_paths>
The number of paths across which to distribute the I/O.
<region_size>
The number of 512-byte sectors in a region. Each region can be redirected
to any of the available paths.
<num_optional_args>
The number of optional arguments. Currently, no optional arguments
are supported and so this must be zero.
<dev_path>
The block device that represents a specific path to the device.
<offset>
The offset of the start of data on the specific <dev_path> (in units
of 512-byte sectors). This number is added to the sector number when
forwarding the request to the specific path. Typically it is zero.
Messages
========
set_region_mappings <index>:<path_nr> [<index>]:<path_nr> [<index>]:<path_nr>...
Modify the region table by specifying which regions are redirected to
which paths.
<index>
The region number (region size was specified in constructor parameters).
If index is omitted, the next region (previous index + 1) is used.
Expressed in hexadecimal (WITHOUT any prefix like 0x).
<path_nr>
The path number in the range 0 ... (<num_paths> - 1).
Expressed in hexadecimal (WITHOUT any prefix like 0x).
R<n>,<m>
This parameter allows repetitive patterns to be loaded quickly. <n> and <m>
are hexadecimal numbers. The last <n> mappings are repeated in the next <m>
slots.
Status
======
No status line is reported.
Example
=======
Assume that you have volumes vg1/switch0 vg1/switch1 vg1/switch2 with
the same size.
Create a switch device with 64kB region size:
dmsetup create switch --table "0 `blockdev --getsize /dev/vg1/switch0`
switch 3 128 0 /dev/vg1/switch0 0 /dev/vg1/switch1 0 /dev/vg1/switch2 0"
Set mappings for the first 7 entries to point to devices switch0, switch1,
switch2, switch0, switch1, switch2, switch1:
dmsetup message switch 0 set_region_mappings 0:0 :1 :2 :0 :1 :2 :1
Set repetitive mapping. This command:
dmsetup message switch 0 set_region_mappings 1000:1 :2 R2,10
is equivalent to:
dmsetup message switch 0 set_region_mappings 1000:1 :2 :1 :2 :1 :2 :1 :2 \
:1 :2 :1 :2 :1 :2 :1 :2 :1 :2

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Introduction
============
This document describes a collection of device-mapper targets that
between them implement thin-provisioning and snapshots.
The main highlight of this implementation, compared to the previous
implementation of snapshots, is that it allows many virtual devices to
be stored on the same data volume. This simplifies administration and
allows the sharing of data between volumes, thus reducing disk usage.
Another significant feature is support for an arbitrary depth of
recursive snapshots (snapshots of snapshots of snapshots ...). The
previous implementation of snapshots did this by chaining together
lookup tables, and so performance was O(depth). This new
implementation uses a single data structure to avoid this degradation
with depth. Fragmentation may still be an issue, however, in some
scenarios.
Metadata is stored on a separate device from data, giving the
administrator some freedom, for example to:
- Improve metadata resilience by storing metadata on a mirrored volume
but data on a non-mirrored one.
- Improve performance by storing the metadata on SSD.
Status
======
These targets are very much still in the EXPERIMENTAL state. Please
do not yet rely on them in production. But do experiment and offer us
feedback. Different use cases will have different performance
characteristics, for example due to fragmentation of the data volume.
If you find this software is not performing as expected please mail
dm-devel@redhat.com with details and we'll try our best to improve
things for you.
Userspace tools for checking and repairing the metadata are under
development.
Cookbook
========
This section describes some quick recipes for using thin provisioning.
They use the dmsetup program to control the device-mapper driver
directly. End users will be advised to use a higher-level volume
manager such as LVM2 once support has been added.
Pool device
-----------
The pool device ties together the metadata volume and the data volume.
It maps I/O linearly to the data volume and updates the metadata via
two mechanisms:
- Function calls from the thin targets
- Device-mapper 'messages' from userspace which control the creation of new
virtual devices amongst other things.
Setting up a fresh pool device
------------------------------
Setting up a pool device requires a valid metadata device, and a
data device. If you do not have an existing metadata device you can
make one by zeroing the first 4k to indicate empty metadata.
dd if=/dev/zero of=$metadata_dev bs=4096 count=1
The amount of metadata you need will vary according to how many blocks
are shared between thin devices (i.e. through snapshots). If you have
less sharing than average you'll need a larger-than-average metadata device.
As a guide, we suggest you calculate the number of bytes to use in the
metadata device as 48 * $data_dev_size / $data_block_size but round it up
to 2MB if the answer is smaller. If you're creating large numbers of
snapshots which are recording large amounts of change, you may find you
need to increase this.
The largest size supported is 16GB: If the device is larger,
a warning will be issued and the excess space will not be used.
Reloading a pool table
----------------------
You may reload a pool's table, indeed this is how the pool is resized
if it runs out of space. (N.B. While specifying a different metadata
device when reloading is not forbidden at the moment, things will go
wrong if it does not route I/O to exactly the same on-disk location as
previously.)
Using an existing pool device
-----------------------------
dmsetup create pool \
--table "0 20971520 thin-pool $metadata_dev $data_dev \
$data_block_size $low_water_mark"
$data_block_size gives the smallest unit of disk space that can be
allocated at a time expressed in units of 512-byte sectors.
$data_block_size must be between 128 (64KB) and 2097152 (1GB) and a
multiple of 128 (64KB). $data_block_size cannot be changed after the
thin-pool is created. People primarily interested in thin provisioning
may want to use a value such as 1024 (512KB). People doing lots of
snapshotting may want a smaller value such as 128 (64KB). If you are
not zeroing newly-allocated data, a larger $data_block_size in the
region of 256000 (128MB) is suggested.
$low_water_mark is expressed in blocks of size $data_block_size. If
free space on the data device drops below this level then a dm event
will be triggered which a userspace daemon should catch allowing it to
extend the pool device. Only one such event will be sent.
Resuming a device with a new table itself triggers an event so the
userspace daemon can use this to detect a situation where a new table
already exceeds the threshold.
A low water mark for the metadata device is maintained in the kernel and
will trigger a dm event if free space on the metadata device drops below
it.
Updating on-disk metadata
-------------------------
On-disk metadata is committed every time a FLUSH or FUA bio is written.
If no such requests are made then commits will occur every second. This
means the thin-provisioning target behaves like a physical disk that has
a volatile write cache. If power is lost you may lose some recent
writes. The metadata should always be consistent in spite of any crash.
If data space is exhausted the pool will either error or queue IO
according to the configuration (see: error_if_no_space). If metadata
space is exhausted or a metadata operation fails: the pool will error IO
until the pool is taken offline and repair is performed to 1) fix any
potential inconsistencies and 2) clear the flag that imposes repair.
Once the pool's metadata device is repaired it may be resized, which
will allow the pool to return to normal operation. Note that if a pool
is flagged as needing repair, the pool's data and metadata devices
cannot be resized until repair is performed. It should also be noted
that when the pool's metadata space is exhausted the current metadata
transaction is aborted. Given that the pool will cache IO whose
completion may have already been acknowledged to upper IO layers
(e.g. filesystem) it is strongly suggested that consistency checks
(e.g. fsck) be performed on those layers when repair of the pool is
required.
Thin provisioning
-----------------
i) Creating a new thinly-provisioned volume.
To create a new thinly- provisioned volume you must send a message to an
active pool device, /dev/mapper/pool in this example.
dmsetup message /dev/mapper/pool 0 "create_thin 0"
Here '0' is an identifier for the volume, a 24-bit number. It's up
to the caller to allocate and manage these identifiers. If the
identifier is already in use, the message will fail with -EEXIST.
ii) Using a thinly-provisioned volume.
Thinly-provisioned volumes are activated using the 'thin' target:
dmsetup create thin --table "0 2097152 thin /dev/mapper/pool 0"
The last parameter is the identifier for the thinp device.
Internal snapshots
------------------
i) Creating an internal snapshot.
Snapshots are created with another message to the pool.
N.B. If the origin device that you wish to snapshot is active, you
must suspend it before creating the snapshot to avoid corruption.
This is NOT enforced at the moment, so please be careful!
dmsetup suspend /dev/mapper/thin
dmsetup message /dev/mapper/pool 0 "create_snap 1 0"
dmsetup resume /dev/mapper/thin
Here '1' is the identifier for the volume, a 24-bit number. '0' is the
identifier for the origin device.
ii) Using an internal snapshot.
Once created, the user doesn't have to worry about any connection
between the origin and the snapshot. Indeed the snapshot is no
different from any other thinly-provisioned device and can be
snapshotted itself via the same method. It's perfectly legal to
have only one of them active, and there's no ordering requirement on
activating or removing them both. (This differs from conventional
device-mapper snapshots.)
Activate it exactly the same way as any other thinly-provisioned volume:
dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 1"
External snapshots
------------------
You can use an external _read only_ device as an origin for a
thinly-provisioned volume. Any read to an unprovisioned area of the
thin device will be passed through to the origin. Writes trigger
the allocation of new blocks as usual.
One use case for this is VM hosts that want to run guests on
thinly-provisioned volumes but have the base image on another device
(possibly shared between many VMs).
You must not write to the origin device if you use this technique!
Of course, you may write to the thin device and take internal snapshots
of the thin volume.
i) Creating a snapshot of an external device
This is the same as creating a thin device.
You don't mention the origin at this stage.
dmsetup message /dev/mapper/pool 0 "create_thin 0"
ii) Using a snapshot of an external device.
Append an extra parameter to the thin target specifying the origin:
dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 0 /dev/image"
N.B. All descendants (internal snapshots) of this snapshot require the
same extra origin parameter.
Deactivation
------------
All devices using a pool must be deactivated before the pool itself
can be.
dmsetup remove thin
dmsetup remove snap
dmsetup remove pool
Reference
=========
'thin-pool' target
------------------
i) Constructor
thin-pool <metadata dev> <data dev> <data block size (sectors)> \
<low water mark (blocks)> [<number of feature args> [<arg>]*]
Optional feature arguments:
skip_block_zeroing: Skip the zeroing of newly-provisioned blocks.
ignore_discard: Disable discard support.
no_discard_passdown: Don't pass discards down to the underlying
data device, but just remove the mapping.
read_only: Don't allow any changes to be made to the pool
metadata.
error_if_no_space: Error IOs, instead of queueing, if no space.
Data block size must be between 64KB (128 sectors) and 1GB
(2097152 sectors) inclusive.
ii) Status
<transaction id> <used metadata blocks>/<total metadata blocks>
<used data blocks>/<total data blocks> <held metadata root>
[no_]discard_passdown ro|rw
transaction id:
A 64-bit number used by userspace to help synchronise with metadata
from volume managers.
used data blocks / total data blocks
If the number of free blocks drops below the pool's low water mark a
dm event will be sent to userspace. This event is edge-triggered and
it will occur only once after each resume so volume manager writers
should register for the event and then check the target's status.
held metadata root:
The location, in blocks, of the metadata root that has been
'held' for userspace read access. '-' indicates there is no
held root.
discard_passdown|no_discard_passdown
Whether or not discards are actually being passed down to the
underlying device. When this is enabled when loading the table,
it can get disabled if the underlying device doesn't support it.
ro|rw
If the pool encounters certain types of device failures it will
drop into a read-only metadata mode in which no changes to
the pool metadata (like allocating new blocks) are permitted.
In serious cases where even a read-only mode is deemed unsafe
no further I/O will be permitted and the status will just
contain the string 'Fail'. The userspace recovery tools
should then be used.
error_if_no_space|queue_if_no_space
If the pool runs out of data or metadata space, the pool will
either queue or error the IO destined to the data device. The
default is to queue the IO until more space is added or the
'no_space_timeout' expires. The 'no_space_timeout' dm-thin-pool
module parameter can be used to change this timeout -- it
defaults to 60 seconds but may be disabled using a value of 0.
iii) Messages
create_thin <dev id>
Create a new thinly-provisioned device.
<dev id> is an arbitrary unique 24-bit identifier chosen by
the caller.
create_snap <dev id> <origin id>
Create a new snapshot of another thinly-provisioned device.
<dev id> is an arbitrary unique 24-bit identifier chosen by
the caller.
<origin id> is the identifier of the thinly-provisioned device
of which the new device will be a snapshot.
delete <dev id>
Deletes a thin device. Irreversible.
set_transaction_id <current id> <new id>
Userland volume managers, such as LVM, need a way to
synchronise their external metadata with the internal metadata of the
pool target. The thin-pool target offers to store an
arbitrary 64-bit transaction id and return it on the target's
status line. To avoid races you must provide what you think
the current transaction id is when you change it with this
compare-and-swap message.
reserve_metadata_snap
Reserve a copy of the data mapping btree for use by userland.
This allows userland to inspect the mappings as they were when
this message was executed. Use the pool's status command to
get the root block associated with the metadata snapshot.
release_metadata_snap
Release a previously reserved copy of the data mapping btree.
'thin' target
-------------
i) Constructor
thin <pool dev> <dev id> [<external origin dev>]
pool dev:
the thin-pool device, e.g. /dev/mapper/my_pool or 253:0
dev id:
the internal device identifier of the device to be
activated.
external origin dev:
an optional block device outside the pool to be treated as a
read-only snapshot origin: reads to unprovisioned areas of the
thin target will be mapped to this device.
The pool doesn't store any size against the thin devices. If you
load a thin target that is smaller than you've been using previously,
then you'll have no access to blocks mapped beyond the end. If you
load a target that is bigger than before, then extra blocks will be
provisioned as and when needed.
If you wish to reduce the size of your thin device and potentially
regain some space then send the 'trim' message to the pool.
ii) Status
<nr mapped sectors> <highest mapped sector>
If the pool has encountered device errors and failed, the status
will just contain the string 'Fail'. The userspace recovery
tools should then be used.

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dm-verity
==========
Device-Mapper's "verity" target provides transparent integrity checking of
block devices using a cryptographic digest provided by the kernel crypto API.
This target is read-only.
Construction Parameters
=======================
<version> <dev> <hash_dev>
<data_block_size> <hash_block_size>
<num_data_blocks> <hash_start_block>
<algorithm> <digest> <salt>
<version>
This is the type of the on-disk hash format.
0 is the original format used in the Chromium OS.
The salt is appended when hashing, digests are stored continuously and
the rest of the block is padded with zeros.
1 is the current format that should be used for new devices.
The salt is prepended when hashing and each digest is
padded with zeros to the power of two.
<dev>
This is the device containing data, the integrity of which needs to be
checked. It may be specified as a path, like /dev/sdaX, or a device number,
<major>:<minor>.
<hash_dev>
This is the device that supplies the hash tree data. It may be
specified similarly to the device path and may be the same device. If the
same device is used, the hash_start should be outside the configured
dm-verity device.
<data_block_size>
The block size on a data device in bytes.
Each block corresponds to one digest on the hash device.
<hash_block_size>
The size of a hash block in bytes.
<num_data_blocks>
The number of data blocks on the data device. Additional blocks are
inaccessible. You can place hashes to the same partition as data, in this
case hashes are placed after <num_data_blocks>.
<hash_start_block>
This is the offset, in <hash_block_size>-blocks, from the start of hash_dev
to the root block of the hash tree.
<algorithm>
The cryptographic hash algorithm used for this device. This should
be the name of the algorithm, like "sha1".
<digest>
The hexadecimal encoding of the cryptographic hash of the root hash block
and the salt. This hash should be trusted as there is no other authenticity
beyond this point.
<salt>
The hexadecimal encoding of the salt value.
Theory of operation
===================
dm-verity is meant to be set up as part of a verified boot path. This
may be anything ranging from a boot using tboot or trustedgrub to just
booting from a known-good device (like a USB drive or CD).
When a dm-verity device is configured, it is expected that the caller
has been authenticated in some way (cryptographic signatures, etc).
After instantiation, all hashes will be verified on-demand during
disk access. If they cannot be verified up to the root node of the
tree, the root hash, then the I/O will fail. This should detect
tampering with any data on the device and the hash data.
Cryptographic hashes are used to assert the integrity of the device on a
per-block basis. This allows for a lightweight hash computation on first read
into the page cache. Block hashes are stored linearly, aligned to the nearest
block size.
Hash Tree
---------
Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
of some data block on disk is calculated. If it is an intermediary node,
the hash of a number of child nodes is calculated.
Each entry in the tree is a collection of neighboring nodes that fit in one
block. The number is determined based on block_size and the size of the
selected cryptographic digest algorithm. The hashes are linearly-ordered in
this entry and any unaligned trailing space is ignored but included when
calculating the parent node.
The tree looks something like:
alg = sha256, num_blocks = 32768, block_size = 4096
[ root ]
/ . . . \
[entry_0] [entry_1]
/ . . . \ . . . \
[entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
/ ... \ / . . . \ / \
blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
On-disk format
==============
The verity kernel code does not read the verity metadata on-disk header.
It only reads the hash blocks which directly follow the header.
It is expected that a user-space tool will verify the integrity of the
verity header.
Alternatively, the header can be omitted and the dmsetup parameters can
be passed via the kernel command-line in a rooted chain of trust where
the command-line is verified.
Directly following the header (and with sector number padded to the next hash
block boundary) are the hash blocks which are stored a depth at a time
(starting from the root), sorted in order of increasing index.
The full specification of kernel parameters and on-disk metadata format
is available at the cryptsetup project's wiki page
http://code.google.com/p/cryptsetup/wiki/DMVerity
Status
======
V (for Valid) is returned if every check performed so far was valid.
If any check failed, C (for Corruption) is returned.
Example
=======
Set up a device:
# dmsetup create vroot --readonly --table \
"0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
"4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
"1234000000000000000000000000000000000000000000000000000000000000"
A command line tool veritysetup is available to compute or verify
the hash tree or activate the kernel device. This is available from
the cryptsetup upstream repository http://code.google.com/p/cryptsetup/
(as a libcryptsetup extension).
Create hash on the device:
# veritysetup format /dev/sda1 /dev/sda2
...
Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
Activate the device:
# veritysetup create vroot /dev/sda1 /dev/sda2 \
4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076

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dm-zero
=======
Device-Mapper's "zero" target provides a block-device that always returns
zero'd data on reads and silently drops writes. This is similar behavior to
/dev/zero, but as a block-device instead of a character-device.
Dm-zero has no target-specific parameters.
One very interesting use of dm-zero is for creating "sparse" devices in
conjunction with dm-snapshot. A sparse device reports a device-size larger
than the amount of actual storage space available for that device. A user can
write data anywhere within the sparse device and read it back like a normal
device. Reads to previously unwritten areas will return a zero'd buffer. When
enough data has been written to fill up the actual storage space, the sparse
device is deactivated. This can be very useful for testing device and
filesystem limitations.
To create a sparse device, start by creating a dm-zero device that's the
desired size of the sparse device. For this example, we'll assume a 10TB
sparse device.
TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2` # 10 TB in sectors
echo "0 $TEN_TERABYTES zero" | dmsetup create zero1
Then create a snapshot of the zero device, using any available block-device as
the COW device. The size of the COW device will determine the amount of real
space available to the sparse device. For this example, we'll assume /dev/sdb1
is an available 10GB partition.
echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
dmsetup create sparse1
This will create a 10TB sparse device called /dev/mapper/sparse1 that has
10GB of actual storage space available. If more than 10GB of data is written
to this device, it will start returning I/O errors.