+++ /dev/null
-=============
- 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
Code Review / stor4nfv.git / blobdiff