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doc: document "readforward" and "readproxy" cache mode

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Signed-off-by: Kefu Chai <kchai@redhat.com>
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Kefu Chai 2015-12-23 13:43:15 +08:00
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doc/rados/operations/cache-tiering.rst
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@ -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might contain out-of-date data provides weak consistency.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.