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| -rw-r--r-- | doc/dev/versions.rst | 42 |
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diff --git a/doc/dev/versions.rst b/doc/dev/versions.rst new file mode 100644 index 00000000..34ed7472 --- /dev/null +++ b/doc/dev/versions.rst @@ -0,0 +1,42 @@ +================== +Public OSD Version +================== + +We maintain two versions on disk: an eversion_t pg_log.head and a +version_t info.user_version. Each object is tagged with both the pg +version and user_version it was last modified with. The PG version is +modified by manipulating OpContext::at_version and then persisting it +to the pg log as transactions, and is incremented in all the places it +used to be. The user_version is modified by manipulating the new +OpContext::user_at_version and is also persisted via the pg log +transactions. +user_at_version is modified only in PrimaryLogPG::prepare_transaction +when the op was a "user modify" (a non-watch write), and the durable +user_version is updated according to the following rules: +1) set user_at_version to the maximum of ctx->new_obs.oi.user_version+1 +and info.last_user_version+1. +2) set user_at_version to the maximum of itself and +ctx->at_version.version. +3) ctx->new_obs.oi.user_version = ctx->user_at_version (to change the +object's user_version) + +This set of update semantics mean that for traditional pools the +user_version will be equal to the past reassert_version, while for +caching pools the object and PG user-version will be able to cross +pools without making a total mess of things. +In order to support old clients, we keep the old reassert_version but +rename it to "bad_replay_version"; we fill it in as before: for writes +it is set to the at_version (and is the proper replay version); for +watches it is set to our user version; for ENOENT replies it is set to +the replay version's epoch but the user_version's version. We also now +fill in the version_t portion of the bad_replay_version on read ops as +well as write ops, which should be fine for all old clients. + +For new clients, we prevent them from reading bad_replay_version and +add two proper members: user_version and replay_version; user_version +is filled in on every operation (reads included) while replay_version +is filled in for writes. + +The objclass function get_current_version() now always returns the +pg->info.last_user_version, which means it is guaranteed to contain +the version of the last user update in the PG (including on reads!). |