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diff --git a/src/ceph/doc/dev/osd_internals/erasure_coding/developer_notes.rst b/src/ceph/doc/dev/osd_internals/erasure_coding/developer_notes.rst new file mode 100644 index 0000000..770ff4a --- /dev/null +++ b/src/ceph/doc/dev/osd_internals/erasure_coding/developer_notes.rst @@ -0,0 +1,223 @@ +============================ +Erasure Code developer notes +============================ + +Introduction +------------ + +Each chapter of this document explains an aspect of the implementation +of the erasure code within Ceph. It is mostly based on examples being +explained to demonstrate how things work. + +Reading and writing encoded chunks from and to OSDs +--------------------------------------------------- + +An erasure coded pool stores each object as K+M chunks. It is divided +into K data chunks and M coding chunks. The pool is configured to have +a size of K+M so that each chunk is stored in an OSD in the acting +set. The rank of the chunk is stored as an attribute of the object. + +Let's say an erasure coded pool is created to use five OSDs ( K+M = +5 ) and sustain the loss of two of them ( M = 2 ). + +When the object *NYAN* containing *ABCDEFGHI* is written to it, the +erasure encoding function splits the content in three data chunks, +simply by dividing the content in three : the first contains *ABC*, +the second *DEF* and the last *GHI*. The content will be padded if the +content length is not a multiple of K. The function also creates two +coding chunks : the fourth with *YXY* and the fifth with *GQC*. Each +chunk is stored in an OSD in the acting set. The chunks are stored in +objects that have the same name ( *NYAN* ) but reside on different +OSDs. The order in which the chunks were created must be preserved and +is stored as an attribute of the object ( shard_t ), in addition to its +name. Chunk *1* contains *ABC* and is stored on *OSD5* while chunk *4* +contains *YXY* and is stored on *OSD3*. + +:: + + +-------------------+ + name | NYAN | + +-------------------+ + content | ABCDEFGHI | + +--------+----------+ + | + | + v + +------+------+ + +---------------+ encode(3,2) +-----------+ + | +--+--+---+---+ | + | | | | | + | +-------+ | +-----+ | + | | | | | + +--v---+ +--v---+ +--v---+ +--v---+ +--v---+ + name | NYAN | | NYAN | | NYAN | | NYAN | | NYAN | + +------+ +------+ +------+ +------+ +------+ + shard | 1 | | 2 | | 3 | | 4 | | 5 | + +------+ +------+ +------+ +------+ +------+ + content | ABC | | DEF | | GHI | | YXY | | QGC | + +--+---+ +--+---+ +--+---+ +--+---+ +--+---+ + | | | | | + | | | | | + | | +--+---+ | | + | | | OSD1 | | | + | | +------+ | | + | | +------+ | | + | +------>| OSD2 | | | + | +------+ | | + | +------+ | | + | | OSD3 |<----+ | + | +------+ | + | +------+ | + | | OSD4 |<--------------+ + | +------+ + | +------+ + +----------------->| OSD5 | + +------+ + + + + +When the object *NYAN* is read from the erasure coded pool, the +decoding function reads three chunks : chunk *1* containing *ABC*, +chunk *3* containing *GHI* and chunk *4* containing *YXY* and rebuild +the original content of the object *ABCDEFGHI*. The decoding function +is informed that the chunks *2* and *5* are missing ( they are called +*erasures* ). The chunk *5* could not be read because the *OSD4* is +*out*. + +The decoding function could be called as soon as three chunks are +read : *OSD2* was the slowest and its chunk does not need to be taken into +account. This optimization is not implemented in Firefly. + +:: + + +-------------------+ + name | NYAN | + +-------------------+ + content | ABCDEFGHI | + +--------+----------+ + ^ + | + | + +------+------+ + | decode(3,2) | + | erasures 2,5| + +-------------->| | + | +-------------+ + | ^ ^ + | | +-----+ + | | | + +--+---+ +------+ +--+---+ +--+---+ + name | NYAN | | NYAN | | NYAN | | NYAN | + +------+ +------+ +------+ +------+ + shard | 1 | | 2 | | 3 | | 4 | + +------+ +------+ +------+ +------+ + content | ABC | | DEF | | GHI | | YXY | + +--+---+ +--+---+ +--+---+ +--+---+ + ^ . ^ ^ + | TOO . | | + | SLOW . +--+---+ | + | ^ | OSD1 | | + | | +------+ | + | | +------+ | + | +-------| OSD2 | | + | +------+ | + | +------+ | + | | OSD3 |-----+ + | +------+ + | +------+ + | | OSD4 | OUT + | +------+ + | +------+ + +------------------| OSD5 | + +------+ + + +Erasure code library +-------------------- + +Using `Reed-Solomon <https://en.wikipedia.org/wiki/Reed_Solomon>`_, +with parameters K+M, object O is encoded by dividing it into chunks O1, +O2, ... OM and computing coding chunks P1, P2, ... PK. Any K chunks +out of the available K+M chunks can be used to obtain the original +object. If data chunk O2 or coding chunk P2 are lost, they can be +repaired using any K chunks out of the K+M chunks. If more than M +chunks are lost, it is not possible to recover the object. + +Reading the original content of object O can be a simple +concatenation of O1, O2, ... OM, because the plugins are using +`systematic codes +<http://en.wikipedia.org/wiki/Systematic_code>`_. Otherwise the chunks +must be given to the erasure code library *decode* method to retrieve +the content of the object. + +Performance depend on the parameters to the encoding functions and +is also influenced by the packet sizes used when calling the encoding +functions ( for Cauchy or Liberation for instance ): smaller packets +means more calls and more overhead. + +Although Reed-Solomon is provided as a default, Ceph uses it via an +`abstract API <https://github.com/ceph/ceph/blob/v0.78/src/erasure-code/ErasureCodeInterface.h>`_ designed to +allow each pool to choose the plugin that implements it using +key=value pairs stored in an `erasure code profile`_. + +.. _erasure code profile: ../../../erasure-coded-pool + +:: + + $ ceph osd erasure-code-profile set myprofile \ + crush-failure-domain=osd + $ ceph osd erasure-code-profile get myprofile + directory=/usr/lib/ceph/erasure-code + k=2 + m=1 + plugin=jerasure + technique=reed_sol_van + crush-failure-domain=osd + $ ceph osd pool create ecpool 12 12 erasure myprofile + +The *plugin* is dynamically loaded from *directory* and expected to +implement the *int __erasure_code_init(char *plugin_name, char *directory)* function +which is responsible for registering an object derived from *ErasureCodePlugin* +in the registry. The `ErasureCodePluginExample <https://github.com/ceph/ceph/blob/v0.78/src/test/erasure-code/ErasureCodePluginExample.cc>`_ plugin reads: + +:: + + ErasureCodePluginRegistry &instance = + ErasureCodePluginRegistry::instance(); + instance.add(plugin_name, new ErasureCodePluginExample()); + +The *ErasureCodePlugin* derived object must provide a factory method +from which the concrete implementation of the *ErasureCodeInterface* +object can be generated. The `ErasureCodePluginExample plugin <https://github.com/ceph/ceph/blob/v0.78/src/test/erasure-code/ErasureCodePluginExample.cc>`_ reads: + +:: + + virtual int factory(const map<std::string,std::string> ¶meters, + ErasureCodeInterfaceRef *erasure_code) { + *erasure_code = ErasureCodeInterfaceRef(new ErasureCodeExample(parameters)); + return 0; + } + +The *parameters* argument is the list of *key=value* pairs that were +set in the erasure code profile, before the pool was created. + +:: + + ceph osd erasure-code-profile set myprofile \ + directory=<dir> \ # mandatory + plugin=jerasure \ # mandatory + m=10 \ # optional and plugin dependant + k=3 \ # optional and plugin dependant + technique=reed_sol_van \ # optional and plugin dependant + +Notes +----- + +If the objects are large, it may be impractical to encode and decode +them in memory. However, when using *RBD* a 1TB device is divided in +many individual 4MB objects and *RGW* does the same. + +Encoding and decoding is implemented in the OSD. Although it could be +implemented client side for read write, the OSD must be able to encode +and decode on its own when scrubbing. diff --git a/src/ceph/doc/dev/osd_internals/erasure_coding/ecbackend.rst b/src/ceph/doc/dev/osd_internals/erasure_coding/ecbackend.rst new file mode 100644 index 0000000..624ec21 --- /dev/null +++ b/src/ceph/doc/dev/osd_internals/erasure_coding/ecbackend.rst @@ -0,0 +1,207 @@ +================================= +ECBackend Implementation Strategy +================================= + +Misc initial design notes +========================= + +The initial (and still true for ec pools without the hacky ec +overwrites debug flag enabled) design for ec pools restricted +EC pools to operations which can be easily rolled back: + +- CEPH_OSD_OP_APPEND: We can roll back an append locally by + including the previous object size as part of the PG log event. +- CEPH_OSD_OP_DELETE: The possibility of rolling back a delete + requires that we retain the deleted object until all replicas have + persisted the deletion event. ErasureCoded backend will therefore + need to store objects with the version at which they were created + included in the key provided to the filestore. Old versions of an + object can be pruned when all replicas have committed up to the log + event deleting the object. +- CEPH_OSD_OP_(SET|RM)ATTR: If we include the prior value of the attr + to be set or removed, we can roll back these operations locally. + +Log entries contain a structure explaining how to locally undo the +operation represented by the operation +(see osd_types.h:TransactionInfo::LocalRollBack). + +PGTemp and Crush +---------------- + +Primaries are able to request a temp acting set mapping in order to +allow an up-to-date OSD to serve requests while a new primary is +backfilled (and for other reasons). An erasure coded pg needs to be +able to designate a primary for these reasons without putting it in +the first position of the acting set. It also needs to be able to +leave holes in the requested acting set. + +Core Changes: + +- OSDMap::pg_to_*_osds needs to separately return a primary. For most + cases, this can continue to be acting[0]. +- MOSDPGTemp (and related OSD structures) needs to be able to specify + a primary as well as an acting set. +- Much of the existing code base assumes that acting[0] is the primary + and that all elements of acting are valid. This needs to be cleaned + up since the acting set may contain holes. + +Distinguished acting set positions +---------------------------------- + +With the replicated strategy, all replicas of a PG are +interchangeable. With erasure coding, different positions in the +acting set have different pieces of the erasure coding scheme and are +not interchangeable. Worse, crush might cause chunk 2 to be written +to an OSD which happens already to contain an (old) copy of chunk 4. +This means that the OSD and PG messages need to work in terms of a +type like pair<shard_t, pg_t> in order to distinguish different pg +chunks on a single OSD. + +Because the mapping of object name to object in the filestore must +be 1-to-1, we must ensure that the objects in chunk 2 and the objects +in chunk 4 have different names. To that end, the objectstore must +include the chunk id in the object key. + +Core changes: + +- The objectstore `ghobject_t needs to also include a chunk id + <https://github.com/ceph/ceph/blob/firefly/src/common/hobject.h#L241>`_ making it more like + tuple<hobject_t, gen_t, shard_t>. +- coll_t needs to include a shard_t. +- The OSD pg_map and similar pg mappings need to work in terms of a + spg_t (essentially + pair<pg_t, shard_t>). Similarly, pg->pg messages need to include + a shard_t +- For client->PG messages, the OSD will need a way to know which PG + chunk should get the message since the OSD may contain both a + primary and non-primary chunk for the same pg + +Object Classes +-------------- + +Reads from object classes will return ENOTSUP on ec pools by invoking +a special SYNC read. + +Scrub +----- + +The main catch, however, for ec pools is that sending a crc32 of the +stored chunk on a replica isn't particularly helpful since the chunks +on different replicas presumably store different data. Because we +don't support overwrites except via DELETE, however, we have the +option of maintaining a crc32 on each chunk through each append. +Thus, each replica instead simply computes a crc32 of its own stored +chunk and compares it with the locally stored checksum. The replica +then reports to the primary whether the checksums match. + +With overwrites, all scrubs are disabled for now until we work out +what to do (see doc/dev/osd_internals/erasure_coding/proposals.rst). + +Crush +----- + +If crush is unable to generate a replacement for a down member of an +acting set, the acting set should have a hole at that position rather +than shifting the other elements of the acting set out of position. + +========= +ECBackend +========= + +MAIN OPERATION OVERVIEW +======================= + +A RADOS put operation can span +multiple stripes of a single object. There must be code that +tessellates the application level write into a set of per-stripe write +operations -- some whole-stripes and up to two partial +stripes. Without loss of generality, for the remainder of this +document we will focus exclusively on writing a single stripe (whole +or partial). We will use the symbol "W" to represent the number of +blocks within a stripe that are being written, i.e., W <= K. + +There are three data flows for handling a write into an EC stripe. The +choice of which of the three data flows to choose is based on the size +of the write operation and the arithmetic properties of the selected +parity-generation algorithm. + +(1) whole stripe is written/overwritten +(2) a read-modify-write operation is performed. + +WHOLE STRIPE WRITE +------------------ + +This is the simple case, and is already performed in the existing code +(for appends, that is). The primary receives all of the data for the +stripe in the RADOS request, computes the appropriate parity blocks +and send the data and parity blocks to their destination shards which +write them. This is essentially the current EC code. + +READ-MODIFY-WRITE +----------------- + +The primary determines which of the K-W blocks are to be unmodified, +and reads them from the shards. Once all of the data is received it is +combined with the received new data and new parity blocks are +computed. The modified blocks are sent to their respective shards and +written. The RADOS operation is acknowledged. + +OSD Object Write and Consistency +-------------------------------- + +Regardless of the algorithm chosen above, writing of the data is a two +phase process: commit and rollforward. The primary sends the log +entries with the operation described (see +osd_types.h:TransactionInfo::(LocalRollForward|LocalRollBack). +In all cases, the "commit" is performed in place, possibly leaving some +information required for a rollback in a write-aside object. The +rollforward phase occurs once all acting set replicas have committed +the commit (sorry, overloaded term) and removes the rollback information. + +In the case of overwrites of exsting stripes, the rollback information +has the form of a sparse object containing the old values of the +overwritten extents populated using clone_range. This is essentially +a place-holder implementation, in real life, bluestore will have an +efficient primitive for this. + +The rollforward part can be delayed since we report the operation as +committed once all replicas have committed. Currently, whenever we +send a write, we also indicate that all previously committed +operations should be rolled forward (see +ECBackend::try_reads_to_commit). If there aren't any in the pipeline +when we arrive at the waiting_rollforward queue, we start a dummy +write to move things along (see the Pipeline section later on and +ECBackend::try_finish_rmw). + +ExtentCache +----------- + +It's pretty important to be able to pipeline writes on the same +object. For this reason, there is a cache of extents written by +cacheable operations. Each extent remains pinned until the operations +referring to it are committed. The pipeline prevents rmw operations +from running until uncacheable transactions (clones, etc) are flushed +from the pipeline. + +See ExtentCache.h for a detailed explanation of how the cache +states correspond to the higher level invariants about the conditions +under which cuncurrent operations can refer to the same object. + +Pipeline +-------- + +Reading src/osd/ExtentCache.h should have given a good idea of how +operations might overlap. There are several states involved in +processing a write operation and an important invariant which +isn't enforced by PrimaryLogPG at a higher level which need to be +managed by ECBackend. The important invariant is that we can't +have uncacheable and rmw operations running at the same time +on the same object. For simplicity, we simply enforce that any +operation which contains an rmw operation must wait until +all in-progress uncacheable operations complete. + +There are improvements to be made here in the future. + +For more details, see ECBackend::waiting_* and +ECBackend::try_<from>_to_<to>. + diff --git a/src/ceph/doc/dev/osd_internals/erasure_coding/jerasure.rst b/src/ceph/doc/dev/osd_internals/erasure_coding/jerasure.rst new file mode 100644 index 0000000..27669a0 --- /dev/null +++ b/src/ceph/doc/dev/osd_internals/erasure_coding/jerasure.rst @@ -0,0 +1,33 @@ +=============== +jerasure plugin +=============== + +Introduction +------------ + +The parameters interpreted by the jerasure plugin are: + +:: + + ceph osd erasure-code-profile set myprofile \ + directory=<dir> \ # plugin directory absolute path + plugin=jerasure \ # plugin name (only jerasure) + k=<k> \ # data chunks (default 2) + m=<m> \ # coding chunks (default 2) + technique=<technique> \ # coding technique + +The coding techniques can be chosen among *reed_sol_van*, +*reed_sol_r6_op*, *cauchy_orig*, *cauchy_good*, *liberation*, +*blaum_roth* and *liber8tion*. + +The *src/erasure-code/jerasure* directory contains the +implementation. It is a wrapper around the code found at +`https://github.com/ceph/jerasure <https://github.com/ceph/jerasure>`_ +and `https://github.com/ceph/gf-complete +<https://github.com/ceph/gf-complete>`_ , pinned to the latest stable +version in *.gitmodules*. These repositories are copies of the +upstream repositories `http://jerasure.org/jerasure/jerasure +<http://jerasure.org/jerasure/jerasure>`_ and +`http://jerasure.org/jerasure/gf-complete +<http://jerasure.org/jerasure/gf-complete>`_ . The difference +between the two, if any, should match pull requests against upstream. diff --git a/src/ceph/doc/dev/osd_internals/erasure_coding/proposals.rst b/src/ceph/doc/dev/osd_internals/erasure_coding/proposals.rst new file mode 100644 index 0000000..52f98e8 --- /dev/null +++ b/src/ceph/doc/dev/osd_internals/erasure_coding/proposals.rst @@ -0,0 +1,385 @@ +: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? |