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diff --git a/doc/rados/operations/erasure-code.rst b/doc/rados/operations/erasure-code.rst new file mode 100644 index 000000000..1dea23c35 --- /dev/null +++ b/doc/rados/operations/erasure-code.rst @@ -0,0 +1,262 @@ +.. _ecpool: + +============= + Erasure code +============= + +By default, Ceph `pools <../pools>`_ are created with the type "replicated". In +replicated-type pools, every object is copied to multiple disks (this +multiple copying is the "replication"). + +In contrast, `erasure-coded <https://en.wikipedia.org/wiki/Erasure_code>`_ +pools use a method of data protection that is different from replication. In +erasure coding, data is broken into fragments of two kinds: data blocks and +parity blocks. If a drive fails or becomes corrupted, the parity blocks are +used to rebuild the data. At scale, erasure coding saves space relative to +replication. + +In this documentation, data blocks are referred to as "data chunks" +and parity blocks are referred to as "encoding chunks". + +Erasure codes are also called "forward error correction codes". The +first forward error correction code was developed in 1950 by Richard +Hamming at Bell Laboratories. + + +Creating a sample erasure coded pool +------------------------------------ + +The simplest erasure coded pool is equivalent to `RAID5 +<https://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5>`_ and +requires at least three hosts: + +.. prompt:: bash $ + + ceph osd pool create ecpool erasure + +:: + + pool 'ecpool' created + +.. prompt:: bash $ + + echo ABCDEFGHI | rados --pool ecpool put NYAN - + rados --pool ecpool get NYAN - + +:: + + ABCDEFGHI + +Erasure code profiles +--------------------- + +The default erasure code profile can sustain the loss of two OSDs. This erasure +code profile is equivalent to a replicated pool of size three, but requires +2TB to store 1TB of data instead of 3TB to store 1TB of data. The default +profile can be displayed with this command: + +.. prompt:: bash $ + + ceph osd erasure-code-profile get default + +:: + + k=2 + m=2 + plugin=jerasure + crush-failure-domain=host + technique=reed_sol_van + +.. note:: + The default erasure-coded pool, the profile of which is displayed here, is + not the same as the simplest erasure-coded pool. + + The default erasure-coded pool has two data chunks (k) and two coding chunks + (m). The profile of the default erasure-coded pool is "k=2 m=2". + + The simplest erasure-coded pool has two data chunks (k) and one coding chunk + (m). The profile of the simplest erasure-coded pool is "k=2 m=1". + +Choosing the right profile is important because the profile cannot be modified +after the pool is created. If you find that you need an erasure-coded pool with +a profile different than the one you have created, you must create a new pool +with a different (and presumably more carefully-considered) profile. When the +new pool is created, all objects from the wrongly-configured pool must be moved +to the newly-created pool. There is no way to alter the profile of a pool after its creation. + +The most important parameters of the profile are *K*, *M* and +*crush-failure-domain* because they define the storage overhead and +the data durability. For example, if the desired architecture must +sustain the loss of two racks with a storage overhead of 67% overhead, +the following profile can be defined: + +.. prompt:: bash $ + + ceph osd erasure-code-profile set myprofile \ + k=3 \ + m=2 \ + crush-failure-domain=rack + ceph osd pool create ecpool erasure myprofile + echo ABCDEFGHI | rados --pool ecpool put NYAN - + rados --pool ecpool get NYAN - + +:: + + ABCDEFGHI + +The *NYAN* object will be divided in three (*K=3*) and two additional +*chunks* will be created (*M=2*). The value of *M* defines how many +OSD can be lost simultaneously without losing any data. The +*crush-failure-domain=rack* will create a CRUSH rule that ensures +no two *chunks* are stored in the same rack. + +.. ditaa:: + +-------------------+ + 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 | + +--+---+ +--+---+ +--+---+ +--+---+ +--+---+ + | | | | | + | | v | | + | | +--+---+ | | + | | | OSD1 | | | + | | +------+ | | + | | | | + | | +------+ | | + | +------>| OSD2 | | | + | +------+ | | + | | | + | +------+ | | + | | OSD3 |<----+ | + | +------+ | + | | + | +------+ | + | | OSD4 |<--------------+ + | +------+ + | + | +------+ + +----------------->| OSD5 | + +------+ + + +More information can be found in the `erasure code profiles +<../erasure-code-profile>`_ documentation. + + +Erasure Coding with Overwrites +------------------------------ + +By default, erasure coded pools only work with uses like RGW that +perform full object writes and appends. + +Since Luminous, partial writes for an erasure coded pool may be +enabled with a per-pool setting. This lets RBD and CephFS store their +data in an erasure coded pool: + +.. prompt:: bash $ + + ceph osd pool set ec_pool allow_ec_overwrites true + +This can only be enabled on a pool residing on bluestore OSDs, since +bluestore's checksumming is used to detect bitrot or other corruption +during deep-scrub. In addition to being unsafe, using filestore with +ec overwrites yields low performance compared to bluestore. + +Erasure coded pools do not support omap, so to use them with RBD and +CephFS you must instruct them to store their data in an ec pool, and +their metadata in a replicated pool. For RBD, this means using the +erasure coded pool as the ``--data-pool`` during image creation: + +.. prompt:: bash $ + + rbd create --size 1G --data-pool ec_pool replicated_pool/image_name + +For CephFS, an erasure coded pool can be set as the default data pool during +file system creation or via `file layouts <../../../cephfs/file-layouts>`_. + + +Erasure coded pool and cache tiering +------------------------------------ + +Erasure coded pools require more resources than replicated pools and +lack some functionalities such as omap. To overcome these +limitations, one can set up a `cache tier <../cache-tiering>`_ +before the erasure coded pool. + +For instance, if the pool *hot-storage* is made of fast storage: + +.. prompt:: bash $ + + ceph osd tier add ecpool hot-storage + ceph osd tier cache-mode hot-storage writeback + ceph osd tier set-overlay ecpool hot-storage + +will place the *hot-storage* pool as tier of *ecpool* in *writeback* +mode so that every write and read to the *ecpool* are actually using +the *hot-storage* and benefit from its flexibility and speed. + +More information can be found in the `cache tiering +<../cache-tiering>`_ documentation. + +Erasure coded pool recovery +--------------------------- +If an erasure coded pool loses some shards, it must recover them from the others. +This generally involves reading from the remaining shards, reconstructing the data, and +writing it to the new peer. +In Octopus, erasure coded pools can recover as long as there are at least *K* shards +available. (With fewer than *K* shards, you have actually lost data!) + +Prior to Octopus, erasure coded pools required at least *min_size* shards to be +available, even if *min_size* is greater than *K*. (We generally recommend min_size +be *K+2* or more to prevent loss of writes and data.) +This conservative decision was made out of an abundance of caution when designing the new pool +mode but also meant pools with lost OSDs but no data loss were unable to recover and go active +without manual intervention to change the *min_size*. + +Glossary +-------- + +*chunk* + when the encoding function is called, it returns chunks of the same + size. Data chunks which can be concatenated to reconstruct the original + object and coding chunks which can be used to rebuild a lost chunk. + +*K* + the number of data *chunks*, i.e. the number of *chunks* in which the + original object is divided. For instance if *K* = 2 a 10KB object + will be divided into *K* objects of 5KB each. + +*M* + the number of coding *chunks*, i.e. the number of additional *chunks* + computed by the encoding functions. If there are 2 coding *chunks*, + it means 2 OSDs can be out without losing data. + + +Table of content +---------------- + +.. toctree:: + :maxdepth: 1 + + erasure-code-profile + erasure-code-jerasure + erasure-code-isa + erasure-code-lrc + erasure-code-shec + erasure-code-clay |