| 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| may be out of date or kept in sync with the copy on the cache device |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. |
| |
| 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. |
| |
| 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). |
| |
| Writeback/writethrough |
| ---------------------- |
| |
| The cache has two modes, writeback and writethrough. |
| |
| 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. |
| |
| 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 occuring 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' |
