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doc: Update dm kernel files.

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v4.0-9804-gdb4fd9c
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Alasdair G Kergon 2015-04-22 15:34:25 +01:00
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38
doc/kernel/cache-policies.txt
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@ -30,28 +30,48 @@ multiqueue
This policy is the default.
The multiqueue policy has two sets of 16 queues: one set for entries
waiting for the cache and another one for those in the cache.
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>' and
'random_threshold <#nr_random_ios>'.
'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
required before a stream is treated as sequential. Once a stream is
considered sequential it will bypass the cache. 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.
Large, sequential I/Os are probably better left on the origin device
since spindles tend to have good sequential I/O bandwidth. The
io_tracker counts contiguous I/Os to try to spot when the I/O is in one
of these sequential modes. But there are use-cases for wanting to
promote sequential blocks to the cache (e.g. fast application startup).
If sequential threshold is set to 0 the sequential I/O detection is
disabled and sequential I/O will no longer implicitly bypass the cache.
Setting the random threshold to 0 does _not_ disable the random I/O
stream detection.
Internally the mq policy determines a promotion threshold. 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
-------

97
doc/kernel/cache.txt
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@ -50,14 +50,16 @@ other parameters detailed later):
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.
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 256k - 1024k.
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
@ -66,10 +68,11 @@ 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).
Writeback/writethrough
----------------------
Cache operating modes
---------------------
The cache has two modes, writeback and writethrough.
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
@ -79,15 +82,38 @@ 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.
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 occuring at any one time. Currently we're not taking any
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.
@ -98,12 +124,11 @@ the default being 204800 sectors (or 100MB).
Updating on-disk metadata
-------------------------
On-disk metadata is committed every time a REQ_SYNC or REQ_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
write cache (the same is true of the thin-provisioning target). If
power is lost you may lose some recent writes. The metadata should
always be consistent in spite of any crash.
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
@ -159,7 +184,7 @@ Constructor
block size : cache unit size in sectors
#feature args : number of feature arguments passed
feature args : writethrough. (The default is writeback.)
feature args : writethrough or passthrough (The default is writeback.)
policy : the replacement policy to use
#policy args : an even number of arguments corresponding to
@ -175,6 +200,13 @@ Optional feature arguments are:
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.
@ -184,13 +216,20 @@ the characteristics of a specific policy, always request it by name.
Status
------
<#used metadata blocks>/<#total metadata blocks> <#read hits> <#read misses>
<#write hits> <#write misses> <#demotions> <#promotions> <#blocks in cache>
<#dirty> <#features> <features>* <#core args> <core args>* <#policy args>
<policy args>*
<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
@ -203,7 +242,6 @@ Status
from the cache
#promotions : Number of times a block has been moved to
the cache
#blocks in cache : Number of blocks resident in the cache
#dirty : Number of blocks in the cache that differ
from the origin
#feature args : Number of feature args to follow
@ -211,9 +249,10 @@ 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 1024
e.g. sequential_threshold
Messages
--------
@ -229,12 +268,28 @@ The message format is:
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/thinp-test-suite
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'

28
doc/kernel/crypt.txt
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@ -4,12 +4,15 @@ 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:
https://gitlab.com/cryptsetup/cryptsetup/wikis/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-ivopts:ivmode).
(In format cipher[:keycount]-chainmode-ivmode[:ivopts]).
Examples:
des
aes-cbc-essiv:sha256
@ -19,7 +22,11 @@ Parameters: <cipher> <key> <iv_offset> <device path> \
<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.
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
@ -44,7 +51,7 @@ Parameters: <cipher> <key> <iv_offset> <device path> \
Otherwise #opt_params is the number of following arguments.
Example of optional parameters section:
1 allow_discards
3 allow_discards same_cpu_crypt submit_from_crypt_cpus
allow_discards
Block discard requests (a.k.a. TRIM) are passed through the crypt device.
@ -56,11 +63,24 @@ allow_discards
used space etc.) if the discarded blocks can be located easily on the
device later.
same_cpu_crypt
Perform encryption using the same cpu that IO was submitted on.
The default is to use an unbound workqueue so that encryption work
is automatically balanced between available CPUs.
submit_from_crypt_cpus
Disable offloading writes to a separate thread after encryption.
There are some situations where offloading write bios from the
encryption threads to a single thread degrades performance
significantly. The default is to offload write bios to the same
thread because it benefits CFQ to have writes submitted using the
same context.
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/
https://gitlab.com/cryptsetup/cryptsetup
[[
#!/bin/sh

108
doc/kernel/era.txt Normal file
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@ -0,0 +1,108 @@
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

140
doc/kernel/log-writes.txt Normal file
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@ -0,0 +1,140 @@
dm-log-writes
=============
This target takes 2 devices, one to pass all IO to normally, and one to log all
of the write operations to. This is intended for file system developers wishing
to verify the integrity of metadata or data as the file system is written to.
There is a log_write_entry written for every WRITE request and the target is
able to take arbitrary data from userspace to insert into the log. The data
that is in the WRITE requests is copied into the log to make the replay happen
exactly as it happened originally.
Log Ordering
============
We log things in order of completion once we are sure the write is no longer in
cache. This means that normal WRITE requests are not actually logged until the
next REQ_FLUSH request. This is to make it easier for userspace to replay the
log in a way that correlates to what is on disk and not what is in cache, to
make it easier to detect improper waiting/flushing.
This works by attaching all WRITE requests to a list once the write completes.
Once we see a REQ_FLUSH request we splice this list onto the request and once
the FLUSH request completes we log all of the WRITEs and then the FLUSH. Only
completed WRITEs, at the time the REQ_FLUSH is issued, are added in order to
simulate the worst case scenario with regard to power failures. Consider the
following example (W means write, C means complete):
W1,W2,W3,C3,C2,Wflush,C1,Cflush
The log would show the following
W3,W2,flush,W1....
Again this is to simulate what is actually on disk, this allows us to detect
cases where a power failure at a particular point in time would create an
inconsistent file system.
Any REQ_FUA requests bypass this flushing mechanism and are logged as soon as
they complete as those requests will obviously bypass the device cache.
Any REQ_DISCARD requests are treated like WRITE requests. Otherwise we would
have all the DISCARD requests, and then the WRITE requests and then the FLUSH
request. Consider the following example:
WRITE block 1, DISCARD block 1, FLUSH
If we logged DISCARD when it completed, the replay would look like this
DISCARD 1, WRITE 1, FLUSH
which isn't quite what happened and wouldn't be caught during the log replay.
Target interface
================
i) Constructor
log-writes <dev_path> <log_dev_path>
dev_path : Device that all of the IO will go to normally.
log_dev_path : Device where the log entries are written to.
ii) Status
<#logged entries> <highest allocated sector>
#logged entries : Number of logged entries
highest allocated sector : Highest allocated sector
iii) Messages
mark <description>
You can use a dmsetup message to set an arbitrary mark in a log.
For example say you want to fsck a file system after every
write, but first you need to replay up to the mkfs to make sure
we're fsck'ing something reasonable, you would do something like
this:
mkfs.btrfs -f /dev/mapper/log
dmsetup message log 0 mark mkfs
<run test>
This would allow you to replay the log up to the mkfs mark and
then replay from that point on doing the fsck check in the
interval that you want.
Every log has a mark at the end labeled "dm-log-writes-end".
Userspace component
===================
There is a userspace tool that will replay the log for you in various ways.
It can be found here: https://github.com/josefbacik/log-writes
Example usage
=============
Say you want to test fsync on your file system. You would do something like
this:
TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc"
dmsetup create log --table "$TABLE"
mkfs.btrfs -f /dev/mapper/log
dmsetup message log 0 mark mkfs
mount /dev/mapper/log /mnt/btrfs-test
<some test that does fsync at the end>
dmsetup message log 0 mark fsync
md5sum /mnt/btrfs-test/foo
umount /mnt/btrfs-test
dmsetup remove log
replay-log --log /dev/sdc --replay /dev/sdb --end-mark fsync
mount /dev/sdb /mnt/btrfs-test
md5sum /mnt/btrfs-test/foo
<verify md5sum's are correct>
Another option is to do a complicated file system operation and verify the file
system is consistent during the entire operation. You could do this with:
TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc"
dmsetup create log --table "$TABLE"
mkfs.btrfs -f /dev/mapper/log
dmsetup message log 0 mark mkfs
mount /dev/mapper/log /mnt/btrfs-test
<fsstress to dirty the fs>
btrfs filesystem balance /mnt/btrfs-test
umount /mnt/btrfs-test
dmsetup remove log
replay-log --log /dev/sdc --replay /dev/sdb --end-mark mkfs
btrfsck /dev/sdb
replay-log --log /dev/sdc --replay /dev/sdb --start-mark mkfs \
--fsck "btrfsck /dev/sdb" --check fua
And that will replay the log until it sees a FUA request, run the fsck command
and if the fsck passes it will replay to the next FUA, until it is completed or
the fsck command exists abnormally.

2
doc/kernel/raid.txt
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@ -222,3 +222,5 @@ Version History
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".

186
doc/kernel/statistics.txt Normal file
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@ -0,0 +1,186 @@
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

138
doc/kernel/switch.txt Normal file
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@ -0,0 +1,138 @@
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

62
doc/kernel/thin-provisioning.txt
View File

@ -99,13 +99,14 @@ Using an existing pool device
$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. 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.
$data_block_size must be the same for the lifetime of the
metadata device.
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
@ -115,6 +116,35 @@ 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
-----------------
@ -234,6 +264,8 @@ i) Constructor
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.
@ -255,10 +287,9 @@ ii) Status
should register for the event and then check the target's status.
held metadata root:
The location, in sectors, of the metadata root that has been
The location, in blocks, of the metadata root that has been
'held' for userspace read access. '-' indicates there is no
held root. This feature is not yet implemented so '-' is
always returned.
held root.
discard_passdown|no_discard_passdown
Whether or not discards are actually being passed down to the
@ -275,6 +306,14 @@ ii) Status
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>
@ -341,9 +380,6 @@ 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>

21
doc/kernel/verity.txt
View File

@ -11,6 +11,7 @@ Construction Parameters
<data_block_size> <hash_block_size>
<num_data_blocks> <hash_start_block>
<algorithm> <digest> <salt>
[<#opt_params> <opt_params>]
<version>
This is the type of the on-disk hash format.
@ -62,6 +63,22 @@ Construction Parameters
<salt>
The hexadecimal encoding of the salt value.
<#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 ignore_corruption
ignore_corruption
Log corrupted blocks, but allow read operations to proceed normally.
restart_on_corruption
Restart the system when a corrupted block is discovered. This option is
not compatible with ignore_corruption and requires user space support to
avoid restart loops.
Theory of operation
===================
@ -125,7 +142,7 @@ block boundary) are the hash blocks which are stored a depth at a time
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
https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
Status
======
@ -142,7 +159,7 @@ Set up a device:
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/
the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
(as a libcryptsetup extension).
Create hash on the device: