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118 lines
4.7 KiB
ReStructuredText
118 lines
4.7 KiB
ReStructuredText
RADOS client protocol
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=====================
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This is very incomplete, but one must start somewhere.
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Basics
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------
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Requests are MOSDOp messages. Replies are MOSDOpReply messages.
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An object request is targetted at an hobject_t, which includes a pool,
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hash value, object name, placement key (usually empty), and snapid.
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The hash value is a 32-bit hash value, normally generated by hashing
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the object name. The hobject_t can be arbitrarily constructed,
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though, with any hash value and name. Note that in the MOSDOp these
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components are spread across several fields and not logically
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assembled in an actual hobject_t member (mainly historical reasons).
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A request can also target a PG. In this case, the *ps* value matches
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a specific PG, the object name is empty, and (hopefully) the ops in
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the request are PG ops.
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Either way, the request ultimately targets a PG, either by using the
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explicit pgid or by folding the hash value onto the current number of
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pgs in the pool. The client sends the request to the primary for the
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assocated PG.
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Each request is assigned a unique tid.
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Resends
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-------
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If there is a connection drop, the client will resend any outstanding
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requets.
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Any time there is a PG mapping change such that the primary changes,
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the client is responsible for resending the request. Note that
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although there may be an interval change from the OSD's perspective
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(triggering PG peering), if the primary doesn't change then the client
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need not resend.
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There are a few exceptions to this rule:
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* There is a last_force_op_resend field in the pg_pool_t in the
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OSDMap. If this changes, then the clients are forced to resend any
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outstanding requests. (This happens when tiering is adjusted, for
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example.)
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* Some requests are such that they are resent on *any* PG interval
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change, as defined by pg_interval_t's is_new_interval() (the same
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criteria used by peering in the OSD).
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* If the PAUSE OSDMap flag is set and unset.
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Each time a request is sent to the OSD the *attempt* field is incremented. The
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first time it is 0, the next 1, etc.
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Backoff
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-------
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Ordinarily the OSD will simply queue any requests it can't immeidately
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process in memory until such time as it can. This can become
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problematic because the OSD limits the total amount of RAM consumed by
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incoming messages: if either of the thresholds for the number of
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messages or the number of bytes is reached, new messages will not be
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read off the network socket, causing backpressure through the network.
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In some cases, though, the OSD knows or expects that a PG or object
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will be unavailable for some time and does not want to consume memory
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by queuing requests. In these cases it can send a MOSDBackoff message
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to the client.
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A backoff request has four properties:
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#. the op code (block, unblock, or ack-block)
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#. *id*, a unique id assigned within this session
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#. hobject_t begin
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#. hobject_t end
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There are two types of backoff: a *PG* backoff will plug all requests
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targetting an entire PG at the client, as described by a range of the
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hash/hobject_t space [begin,end), while an *object* backoff will plug
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all requests targetting a single object (begin == end).
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When the client receives a *block* backoff message, it is now
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responsible for *not* sending any requests for hobject_ts described by
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the backoff. The backoff remains in effect until the backoff is
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cleared (via an 'unblock' message) or the OSD session is closed. A
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*ack_block* message is sent back to the OSD immediately to acknowledge
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receipt of the backoff.
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When an unblock is
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received, it will reference a specific id that the client previous had
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blocked. However, the range described by the unblock may be smaller
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than the original range, as the PG may have split on the OSD. The unblock
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should *only* unblock the range specified in the unblock message. Any requests
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that fall within the unblock request range are reexamined and, if no other
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installed backoff applies, resent.
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On the OSD, Backoffs are also tracked across ranges of the hash space, and
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exist in three states:
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#. new
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#. acked
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#. deleting
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A newly installed backoff is set to *new* and a message is sent to the
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client. When the *ack-block* message is received it is changed to the
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*acked* state. The OSD may process other messages from the client that
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are covered by the backoff in the *new* state, but once the backoff is
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*acked* it should never see a blocked request unless there is a bug.
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If the OSD wants to a remove a backoff in the *acked* state it can
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simply remove it and notify the client. If the backoff is in the
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*new* state it must move it to the *deleting* state and continue to
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use it to discard client requests until the *ack-block* message is
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received, at which point it can finally be removed. This is necessary to
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preserve the order of operations processed by the OSD.
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