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-:orphan:
-
-=================================
-Proposed Next Steps for ECBackend
-=================================
-
-PARITY-DELTA-WRITE
-------------------
-
-RMW operations current require 4 network hops (2 round trips). In
-principle, for some codes, we can reduce this to 3 by sending the
-update to the replicas holding the data blocks and having them
-compute a delta to forward onto the parity blocks.
-
-The primary reads the current values of the "W" blocks and then uses
-the new values of the "W" blocks to compute parity-deltas for each of
-the parity blocks. The W blocks and the parity delta-blocks are sent
-to their respective shards.
-
-The choice of whether to use a read-modify-write or a
-parity-delta-write is complex policy issue that is TBD in the details
-and is likely to be heavily dependant on the computational costs
-associated with a parity-delta vs. a regular parity-generation
-operation. However, it is believed that the parity-delta scheme is
-likely to be the preferred choice, when available.
-
-The internal interface to the erasure coding library plug-ins needs to
-be extended to support the ability to query if parity-delta
-computation is possible for a selected algorithm as well as an
-interface to the actual parity-delta computation algorithm when
-available.
-
-Stripe Cache
-------------
-
-It may be a good idea to extend the current ExtentCache usage to
-cache some data past when the pinning operation releases it.
-One application pattern that is important to optimize is the small
-block sequential write operation (think of the journal of a journaling
-file system or a database transaction log). Regardless of the chosen
-redundancy algorithm, it is advantageous for the primary to
-retain/buffer recently read/written portions of a stripe in order to
-reduce network traffic. The dynamic contents of this cache may be used
-in the determination of whether a read-modify-write or a
-parity-delta-write is performed. The sizing of this cache is TBD, but
-we should plan on allowing at least a few full stripes per active
-client. Limiting the cache occupancy on a per-client basis will reduce
-the noisy neighbor problem.
-
-Recovery and Rollback Details
-=============================
-
-Implementing a Rollback-able Prepare Operation
-----------------------------------------------
-
-The prepare operation is implemented at each OSD through a simulation
-of a versioning or copy-on-write capability for modifying a portion of
-an object.
-
-When a prepare operation is performed, the new data is written into a
-temporary object. The PG log for the
-operation will contain a reference to the temporary object so that it
-can be located for recovery purposes as well as a record of all of the
-shards which are involved in the operation.
-
-In order to avoid fragmentation (and hence, future read performance),
-creation of the temporary object needs special attention. The name of
-the temporary object affects its location within the KV store. Right
-now its unclear whether it's desirable for the name to locate near the
-base object or whether a separate subset of keyspace should be used
-for temporary objects. Sam believes that colocation with the base
-object is preferred (he suggests using the generation counter of the
-ghobject for temporaries). Whereas Allen believes that using a
-separate subset of keyspace is desirable since these keys are
-ephemeral and we don't want to actually colocate them with the base
-object keys. Perhaps some modeling here can help resolve this
-issue. The data of the temporary object wants to be located as close
-to the data of the base object as possible. This may be best performed
-by adding a new ObjectStore creation primitive that takes the base
-object as an addtional parameter that is a hint to the allocator.
-
-Sam: I think that the short lived thing may be a red herring. We'll
-be updating the donor and primary objects atomically, so it seems like
-we'd want them adjacent in the key space, regardless of the donor's
-lifecycle.
-
-The apply operation moves the data from the temporary object into the
-correct position within the base object and deletes the associated
-temporary object. This operation is done using a specialized
-ObjectStore primitive. In the current ObjectStore interface, this can
-be done using the clonerange function followed by a delete, but can be
-done more efficiently with a specialized move primitive.
-Implementation of the specialized primitive on FileStore can be done
-by copying the data. Some file systems have extensions that might also
-be able to implement this operation (like a defrag API that swaps
-chunks between files). It is expected that NewStore will be able to
-support this efficiently and natively (It has been noted that this
-sequence requires that temporary object allocations, which tend to be
-small, be efficiently converted into blocks for main objects and that
-blocks that were formerly inside of main objects must be reusable with
-minimal overhead)
-
-The prepare and apply operations can be separated arbitrarily in
-time. If a read operation accesses an object that has been altered by
-a prepare operation (but without a corresponding apply operation) it
-must return the data after the prepare operation. This is done by
-creating an in-memory database of objects which have had a prepare
-operation without a corresponding apply operation. All read operations
-must consult this in-memory data structure in order to get the correct
-data. It should explicitly recognized that it is likely that there
-will be multiple prepare operations against a single base object and
-the code must handle this case correctly. This code is implemented as
-a layer between ObjectStore and all existing readers. Annoyingly,
-we'll want to trash this state when the interval changes, so the first
-thing that needs to happen after activation is that the primary and
-replicas apply up to last_update so that the empty cache will be
-correct.
-
-During peering, it is now obvious that an unapplied prepare operation
-can easily be rolled back simply by deleting the associated temporary
-object and removing that entry from the in-memory data structure.
-
-Partial Application Peering/Recovery modifications
---------------------------------------------------
-
-Some writes will be small enough to not require updating all of the
-shards holding data blocks. For write amplification minization
-reasons, it would be best to avoid writing to those shards at all,
-and delay even sending the log entries until the next write which
-actually hits that shard.
-
-The delaying (buffering) of the transmission of the prepare and apply
-operations for witnessing OSDs creates new situations that peering
-must handle. In particular the logic for determining the authoritative
-last_update value (and hence the selection of the OSD which has the
-authoritative log) must be modified to account for the valid but
-missing (i.e., delayed/buffered) pglog entries to which the
-authoritative OSD was only a witness to.
-
-Because a partial write might complete without persisting a log entry
-on every replica, we have to do a bit more work to determine an
-authoritative last_update. The constraint (as with a replicated PG)
-is that last_update >= the most recent log entry for which a commit
-was sent to the client (call this actual_last_update). Secondarily,
-we want last_update to be as small as possible since any log entry
-past actual_last_update (we do not apply a log entry until we have
-sent the commit to the client) must be able to be rolled back. Thus,
-the smaller a last_update we choose, the less recovery will need to
-happen (we can always roll back, but rolling a replica forward may
-require an object rebuild). Thus, we will set last_update to 1 before
-the oldest log entry we can prove cannot have been committed. In
-current master, this is simply the last_update of the shortest log
-from that interval (because that log did not persist any entry past
-that point -- a precondition for sending a commit to the client). For
-this design, we must consider the possibility that any log is missing
-at its head log entries in which it did not participate. Thus, we
-must determine the most recent interval in which we went active
-(essentially, this is what find_best_info currently does). We then
-pull the log from each live osd from that interval back to the minimum
-last_update among them. Then, we extend all logs from the
-authoritative interval until each hits an entry in which it should
-have participated, but did not record. The shortest of these extended
-logs must therefore contain any log entry for which we sent a commit
-to the client -- and the last entry gives us our last_update.
-
-Deep scrub support
-------------------
-
-The simple answer here is probably our best bet. EC pools can't use
-the omap namespace at all right now. The simplest solution would be
-to take a prefix of the omap space and pack N M byte L bit checksums
-into each key/value. The prefixing seems like a sensible precaution
-against eventually wanting to store something else in the omap space.
-It seems like any write will need to read at least the blocks
-containing the modified range. However, with a code able to compute
-parity deltas, we may not need to read a whole stripe. Even without
-that, we don't want to have to write to blocks not participating in
-the write. Thus, each shard should store checksums only for itself.
-It seems like you'd be able to store checksums for all shards on the
-parity blocks, but there may not be distinguished parity blocks which
-are modified on all writes (LRC or shec provide two examples). L
-should probably have a fixed number of options (16, 32, 64?) and be
-configurable per-pool at pool creation. N, M should be likewise be
-configurable at pool creation with sensible defaults.
-
-We need to handle online upgrade. I think the right answer is that
-the first overwrite to an object with an append only checksum
-removes the append only checksum and writes in whatever stripe
-checksums actually got written. The next deep scrub then writes
-out the full checksum omap entries.
-
-RADOS Client Acknowledgement Generation Optimization
-====================================================
-
-Now that the recovery scheme is understood, we can discuss the
-generation of of the RADOS operation acknowledgement (ACK) by the
-primary ("sufficient" from above). It is NOT required that the primary
-wait for all shards to complete their respective prepare
-operations. Using our example where the RADOS operations writes only
-"W" chunks of the stripe, the primary will generate and send W+M
-prepare operations (possibly including a send-to-self). The primary
-need only wait for enough shards to be written to ensure recovery of
-the data, Thus after writing W + M chunks you can afford the lost of M
-chunks. Hence the primary can generate the RADOS ACK after W+M-M => W
-of those prepare operations are completed.
-
-Inconsistent object_info_t versions
-===================================
-
-A natural consequence of only writing the blocks which actually
-changed is that we don't want to update the object_info_t of the
-objects which didn't. I actually think it would pose a problem to do
-so: pg ghobject namespaces are generally large, and unless the osd is
-seeing a bunch of overwrites on a small set of objects, I'd expect
-each write to be far enough apart in the backing ghobject_t->data
-mapping to each constitute a random metadata update. Thus, we have to
-accept that not every shard will have the current version in its
-object_info_t. We can't even bound how old the version on a
-particular shard will happen to be. In particular, the primary does
-not necessarily have the current version. One could argue that the
-parity shards would always have the current version, but not every
-code necessarily has designated parity shards which see every write
-(certainly LRC, iirc shec, and even with a more pedestrian code, it
-might be desirable to rotate the shards based on object hash). Even
-if you chose to designate a shard as witnessing all writes, the pg
-might be degraded with that particular shard missing. This is a bit
-tricky, currently reads and writes implicitely return the most recent
-version of the object written. On reads, we'd have to read K shards
-to answer that question. We can get around that by adding a "don't
-tell me the current version" flag. Writes are more problematic: we
-need an object_info from the most recent write in order to form the
-new object_info and log_entry.
-
-A truly terrifying option would be to eliminate version and
-prior_version entirely from the object_info_t. There are a few
-specific purposes it serves:
-
-#. On OSD startup, we prime the missing set by scanning backwards
- from last_update to last_complete comparing the stored object's
- object_info_t to the version of most recent log entry.
-#. During backfill, we compare versions between primary and target
- to avoid some pushes. We use it elsewhere as well
-#. While pushing and pulling objects, we verify the version.
-#. We return it on reads and writes and allow the librados user to
- assert it atomically on writesto allow the user to deal with write
- races (used extensively by rbd).
-
-Case (3) isn't actually essential, just convenient. Oh well. (4)
-is more annoying. Writes are easy since we know the version. Reads
-are tricky because we may not need to read from all of the replicas.
-Simplest solution is to add a flag to rados operations to just not
-return the user version on read. We can also just not support the
-user version assert on ec for now (I think? Only user is rgw bucket
-indices iirc, and those will always be on replicated because they use
-omap).
-
-We can avoid (1) by maintaining the missing set explicitely. It's
-already possible for there to be a missing object without a
-corresponding log entry (Consider the case where the most recent write
-is to an object which has not been updated in weeks. If that write
-becomes divergent, the written object needs to be marked missing based
-on the prior_version which is not in the log.) THe PGLog already has
-a way of handling those edge cases (see divergent_priors). We'd
-simply expand that to contain the entire missing set and maintain it
-atomically with the log and the objects. This isn't really an
-unreasonable option, the addiitonal keys would be fewer than the
-existing log keys + divergent_priors and aren't updated in the fast
-write path anyway.
-
-The second case is a bit trickier. It's really an optimization for
-the case where a pg became not in the acting set long enough for the
-logs to no longer overlap but not long enough for the PG to have
-healed and removed the old copy. Unfortunately, this describes the
-case where a node was taken down for maintenance with noout set. It's
-probably not acceptable to re-backfill the whole OSD in such a case,
-so we need to be able to quickly determine whether a particular shard
-is up to date given a valid acting set of other shards.
-
-Let ordinary writes which do not change the object size not touch the
-object_info at all. That means that the object_info version won't
-match the pg log entry version. Include in the pg_log_entry_t the
-current object_info version as well as which shards participated (as
-mentioned above). In addition to the object_info_t attr, record on
-each shard s a vector recording for each other shard s' the most
-recent write which spanned both s and s'. Operationally, we maintain
-an attr on each shard containing that vector. A write touching S
-updates the version stamp entry for each shard in S on each shard in
-S's attribute (and leaves the rest alone). If we have a valid acting
-set during backfill, we must have a witness of every write which
-completed -- so taking the max of each entry over all of the acting
-set shards must give us the current version for each shard. During
-recovery, we set the attribute on the recovery target to that max
-vector (Question: with LRC, we may not need to touch much of the
-acting set to recover a particular shard -- can we just use the max of
-the shards we used to recovery, or do we need to grab the version
-vector from the rest of the acting set as well? I'm not sure, not a
-big deal anyway, I think).
-
-The above lets us perform blind writes without knowing the current
-object version (log entry version, that is) while still allowing us to
-avoid backfilling up to date objects. The only catch is that our
-backfill scans will can all replicas, not just the primary and the
-backfill targets.
-
-It would be worth adding into scrub the ability to check the
-consistency of the gathered version vectors -- probably by just
-taking 3 random valid subsets and verifying that they generate
-the same authoritative version vector.
-
-Implementation Strategy
-=======================
-
-It goes without saying that it would be unwise to attempt to do all of
-this in one massive PR. It's also not a good idea to merge code which
-isn't being tested. To that end, it's worth thinking a bit about
-which bits can be tested on their own (perhaps with a bit of temporary
-scaffolding).
-
-We can implement the overwrite friendly checksumming scheme easily
-enough with the current implementation. We'll want to enable it on a
-per-pool basis (probably using a flag which we'll later repurpose for
-actual overwrite support). We can enable it in some of the ec
-thrashing tests in the suite. We can also add a simple test
-validating the behavior of turning it on for an existing ec pool
-(later, we'll want to be able to convert append-only ec pools to
-overwrite ec pools, so that test will simply be expanded as we go).
-The flag should be gated by the experimental feature flag since we
-won't want to support this as a valid configuration -- testing only.
-We need to upgrade append only ones in place during deep scrub.
-
-Similarly, we can implement the unstable extent cache with the current
-implementation, it even lets us cut out the readable ack the replicas
-send to the primary after the commit which lets it release the lock.
-Same deal, implement, gate with experimental flag, add to some of the
-automated tests. I don't really see a reason not to use the same flag
-as above.
-
-We can certainly implement the move-range primitive with unit tests
-before there are any users. Adding coverage to the existing
-objectstore tests would suffice here.
-
-Explicit missing set can be implemented now, same deal as above --
-might as well even use the same feature bit.
-
-The TPC protocol outlined above can actually be implemented an append
-only EC pool. Same deal as above, can even use the same feature bit.
-
-The RADOS flag to suppress the read op user version return can be
-implemented immediately. Mostly just needs unit tests.
-
-The version vector problem is an interesting one. For append only EC
-pools, it would be pointless since all writes increase the size and
-therefore update the object_info. We could do it for replicated pools
-though. It's a bit silly since all "shards" see all writes, but it
-would still let us implement and partially test the augmented backfill
-code as well as the extra pg log entry fields -- this depends on the
-explicit pg log entry branch having already merged. It's not entirely
-clear to me that this one is worth doing seperately. It's enough code
-that I'd really prefer to get it done independently, but it's also a
-fair amount of scaffolding that will be later discarded.
-
-PGLog entries need to be able to record the participants and log
-comparison needs to be modified to extend logs with entries they
-wouldn't have witnessed. This logic should be abstracted behind
-PGLog so it can be unittested -- that would let us test it somewhat
-before the actual ec overwrites code merges.
-
-Whatever needs to happen to the ec plugin interface can probably be
-done independently of the rest of this (pending resolution of
-questions below).
-
-The actual nuts and bolts of performing the ec overwrite it seems to
-me can't be productively tested (and therefore implemented) until the
-above are complete, so best to get all of the supporting code in
-first.
-
-Open Questions
-==============
-
-Is there a code we should be using that would let us compute a parity
-delta without rereading and reencoding the full stripe? If so, is it
-the kind of thing we need to design for now, or can it be reasonably
-put off?
-
-What needs to happen to the EC plugin interface?