From 6b6b38163b7742d97d21457cf38bdcc9bde5ae1a Mon Sep 17 00:00:00 2001 From: Sage Weil Date: Wed, 11 May 2016 08:15:53 -0400 Subject: [PATCH] doc/rados: drop description of cache modes we don't recommend Signed-off-by: Sage Weil --- doc/rados/operations/cache-tiering.rst | 36 +++++--------------------- 1 file changed, 7 insertions(+), 29 deletions(-) diff --git a/doc/rados/operations/cache-tiering.rst b/doc/rados/operations/cache-tiering.rst index f7fadd06b6f..8b1731c71a0 100644 --- a/doc/rados/operations/cache-tiering.rst +++ b/doc/rados/operations/cache-tiering.rst @@ -55,35 +55,13 @@ configure how this migration takes place. There are two main scenarios: data becomes inactive. This is ideal for mutable data (e.g., photo/video editing, transactional data, etc.). -- **Read-only Mode:** When admins configure tiers with ``readonly`` mode, Ceph - clients write data to the backing tier. On read, Ceph copies the requested - object(s) from the backing tier to the cache tier. Stale objects get removed - from the cache tier based on the defined policy. This approach is ideal - for immutable data (e.g., presenting pictures/videos on a social network, - DNA data, X-Ray imaging, etc.), because reading data from a cache pool that - might contain out-of-date data provides weak consistency. Do not use - ``readonly`` mode for mutable data. - -And the modes above are accomodated to adapt different configurations: - -- **Read-forward Mode:** this mode is the same as the ``writeback`` mode - when serving write requests. But when Ceph clients is trying to read objects - not yet copied to the cache tier, Ceph **forward** them to the backing tier by - replying with a "redirect" message. And the clients will instead turn to the - backing tier for the data. If the read performance of the backing tier is on - a par with that of its cache tier, while its write performance or endurance - falls far behind, this mode might be a better choice. - -- **Read-proxy Mode:** this mode is similar to ``readforward`` mode: both - of them do not promote/copy the data when the requested object does not - exist in the cache tier. But instead of redirecting the Ceph clients to the - backing tier when cache misses, the cache tier reads from the backing tier - on behalf of the clients. Under some circumstances, this mode can help to - reduce the latency. - -Since all Ceph clients can use cache tiering, it has the potential to -improve I/O performance for Ceph Block Devices, Ceph Object Storage, -the Ceph Filesystem and native bindings. +- **Read-proxy Mode:** This mode will use any objects that already exist + in the cache tier, but if an object is not present in the cache the request + will be proxied to the base tier. This is useful for transitioning from + ``writeback`` mode to a disabled cache as it allows the workload to function + properly while the cache is drained, without adding any new objects to the + cache. + Setting Up Pools -- 2.39.5