From d821acada39937b9dacf87614c924114adea8a58 Mon Sep 17 00:00:00 2001 From: Kefu Chai Date: Wed, 23 Dec 2015 13:43:15 +0800 Subject: [PATCH] doc: document "readforward" and "readproxy" cache mode Signed-off-by: Kefu Chai --- doc/rados/operations/cache-tiering.rst | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/doc/rados/operations/cache-tiering.rst b/doc/rados/operations/cache-tiering.rst index bde4267b56b..daa66c08946 100644 --- a/doc/rados/operations/cache-tiering.rst +++ b/doc/rados/operations/cache-tiering.rst @@ -64,6 +64,23 @@ configure how this migration takes place. There are two main scenarios: might contain out-of-date data provides weak consistency.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. -- 2.47.3