9614e2ba91
In commit f8350daf7a ("dm cache: tune migration throttling") the
value for DEFAULT_MIGRATION_THRESHOLD was decreased from 204800 to
2048. Edit device-mapper/cache.txt to reflect the correct default
value for migration_threshold.
Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
308 lines
12 KiB
Text
308 lines
12 KiB
Text
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may be out of date or kept in sync with the copy on the cache device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
|
|
sectors (32KB) and 2097152 sectors (1GB) and a multiple of 64 sectors (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 2048 sectors (1MB).
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
metadata2 : use version 2 of the metadata. This stores the dirty bits
|
|
in a separate btree, which improves speed of shutting
|
|
down the cache.
|
|
|
|
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>*
|
|
<cache metadata mode>
|
|
|
|
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
|
|
cache metadata mode : ro if read-only, rw if read-write
|
|
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.
|
|
needs_check : 'needs_check' if set, '-' if not set
|
|
A metadata operation has failed, resulting in the needs_check
|
|
flag being set in the metadata's superblock. The metadata
|
|
device must be deactivated and checked/repaired before the
|
|
cache can be made fully operational again. '-' indicates
|
|
needs_check is not set.
|
|
|
|
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 hexadecimal 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'
|