path: root/Documentation/device-mapper/cache.txt

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may be out of date or kept in sync with the copy on the cache device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.  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, the cache device can 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.

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 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.

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.  (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.

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
------

<#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>*

#used metadata blocks    : Number of metadata blocks used
#total metadata blocks   : Total number of metadata 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
#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
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 args             : Number of policy arguments to follow (must be even)
policy args              : Key/value pairs
			     e.g. 'sequential_threshold 1024

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

Examples
========

The test suite can be found here:

https://github.com/jthornber/thinp-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'