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https://github.com/ceph/ceph
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42 lines
2.0 KiB
ReStructuredText
42 lines
2.0 KiB
ReStructuredText
==============
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Public OSD Version
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==============
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We maintain two versions on disk: an eversion_t pg_log.head and a
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version_t info.user_version. Each object is tagged with both the pg
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version and user_version it was last modified with. The PG version is
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modified by manipulating OpContext::at_version and then persisting it
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to the pg log as transactions, and is incremented in all the places it
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used to be. The user_version is modified by manipulating the new
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OpContext::user_at_version and is also persisted via the pg log
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transactions.
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user_at_version is modified only in ReplicatedPG::prepare_transaction
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when the op was a "user modify" (a non-watch write), and the durable
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user_version is updated according to the following rules:
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1) increment user_at_version
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2) set user_at_version to the maximum of itself and
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ctx->at_version.version.
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3) ctx->new_obs.oi.user_version = ctx->user_at_version (to change the
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object's user_version)
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This set of update semantics mean that for traditional pools the
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user_version will be equal to the past reassert_version, while for
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caching pools the object and PG user-version will be able to cross
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pools without making a total mess of things.
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In order to support old clients, we keep the old reassert_version but
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rename it to "bad_replay_version"; we fill it in as before: for writes
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it is set to the at_version (and is the proper replay version); for
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watches it is set to our user version; for ENOENT replies it is set to
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the replay version's epoch but the user_version's version. We also now
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fill in the version_t portion of the bad_replay_version on read ops as
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well as write ops, which should be fine for all old clients.
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For new clients, we prevent them from reading bad_replay_version and
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add two proper members: user_version and replay_version; user_version
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is filled in on every operation (reads included) while replay_version
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is filled in for writes.
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The objclass function get_current_version() now always returns the
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user_at_version, which means it is guaranteed to contain the version
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of the last user update in the PG (including on reads!).
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