diff options
Diffstat (limited to 'src/ceph/doc/rados/operations/erasure-code.rst')
| -rw-r--r-- | src/ceph/doc/rados/operations/erasure-code.rst | 195 |
1 files changed, 0 insertions, 195 deletions
diff --git a/src/ceph/doc/rados/operations/erasure-code.rst b/src/ceph/doc/rados/operations/erasure-code.rst deleted file mode 100644 index 6ec5a09..0000000 --- a/src/ceph/doc/rados/operations/erasure-code.rst +++ /dev/null @@ -1,195 +0,0 @@ -============= - Erasure code -============= - -A Ceph pool is associated to a type to sustain the loss of an OSD -(i.e. a disk since most of the time there is one OSD per disk). The -default choice when `creating a pool <../pools>`_ is *replicated*, -meaning every object is copied on multiple disks. The `Erasure Code -<https://en.wikipedia.org/wiki/Erasure_code>`_ pool type can be used -instead to save space. - -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:: - - $ ceph osd pool create ecpool 12 12 erasure - pool 'ecpool' created - $ echo ABCDEFGHI | rados --pool ecpool put NYAN - - $ rados --pool ecpool get NYAN - - ABCDEFGHI - -.. note:: the 12 in *pool create* stands for - `the number of placement groups <../pools>`_. - -Erasure code profiles ---------------------- - -The default erasure code profile sustains the loss of a single OSD. It -is equivalent to a replicated pool of size two but requires 1.5TB -instead of 2TB to store 1TB of data. The default profile can be -displayed with:: - - $ ceph osd erasure-code-profile get default - k=2 - m=1 - plugin=jerasure - crush-failure-domain=host - technique=reed_sol_van - -Choosing the right profile is important because it cannot be modified -after the pool is created: a new pool with a different profile needs -to be created and all objects from the previous pool moved to the new. - -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 instance, if the desired architecture must -sustain the loss of two racks with a storage overhead of 40% overhead, -the following profile can be defined:: - - $ ceph osd erasure-code-profile set myprofile \ - k=3 \ - m=2 \ - crush-failure-domain=rack - $ ceph osd pool create ecpool 12 12 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 ruleset 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:: - - 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:: - - rbd create --size 1G --data-pool ec_pool replicated_pool/image_name - -For Cephfs, using an erasure coded pool means setting that pool in -a `file layout <../../../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:: - - $ 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. - -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 |