mirror of
https://github.com/ceph/ceph
synced 2024-12-14 07:25:50 +00:00
73 lines
1.9 KiB
ReStructuredText
73 lines
1.9 KiB
ReStructuredText
Cache pool
|
|
==========
|
|
|
|
Purpose
|
|
-------
|
|
|
|
Use a pool of fast storage devices (probably SSDs) and use it as a
|
|
cache for an existing larger pool.
|
|
|
|
We should be able to create and add a cache pool to an existing pool
|
|
of data, and later remove it, without disrupting service or migrating
|
|
data around.
|
|
|
|
Use cases
|
|
---------
|
|
|
|
Read-write pool, writeback
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
We have an existing data pool and put a fast cache pool "in front" of it. Writes will
|
|
go to the cache pool and immediately ack. We flush them back to the data pool based on
|
|
some policy.
|
|
|
|
Read-only pool, weak consistency
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
We have an existing data pool and add one or more read-only cache
|
|
pools. We copy data to the cache pool(s) on read. Writes are
|
|
forwarded to the original data pool. Stale data is expired from the
|
|
cache pools based on some as-yet undetermined policy.
|
|
|
|
This is likely only useful for specific applications with specific
|
|
data access patterns. It may be a match for rgw, for example.
|
|
|
|
|
|
Interface
|
|
---------
|
|
|
|
Set up a read/write cache pool foo-hot for pool foo::
|
|
|
|
ceph osd tier add foo foo-hot
|
|
ceph osd tier cache-mode foo-hot writeback
|
|
|
|
Direct all traffic for foo to foo-hot::
|
|
|
|
ceph osd tier set-overlay foo foo-hot
|
|
|
|
Set the target size and enable the tiering agent for foo-hit::
|
|
|
|
ceph osd pool set foo-hot hit_set_type bloom
|
|
ceph osd pool set foo-hot hit_set_count 1
|
|
ceph osd pool set foo-hot hit_set_period 3600 # 1 hour
|
|
ceph osd pool set foo-hot target_max_bytes 1000000000000 # 1 TB
|
|
|
|
Drain the cache in preparation for turning it off::
|
|
|
|
ceph osd tier cache-mode foo-hot forward
|
|
rados -p foo-hot cache-flush-evict-all
|
|
|
|
When cache pool is finally empty, disable it::
|
|
|
|
ceph osd tier remove-overlay foo
|
|
ceph osd tier remove foo foo-hot
|
|
|
|
Read-only pools with lazy consistency::
|
|
|
|
ceph osd tier add foo foo-east
|
|
ceph osd tier cache-mode foo-east readonly
|
|
ceph osd tier add foo foo-west
|
|
ceph osd tier cache-mode foo-west readonly
|
|
|
|
|