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[stor4nfv.git] / src / ceph / doc / dev / osd_internals / erasure_coding.rst

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-==============================
-Erasure Coded Placement Groups
-==============================
-
-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.
-
-*chunk rank*
-   the index of a chunk when returned by the encoding function. The
-   rank of the first chunk is 0, the rank of the second chunk is 1
-   etc.
-
-*stripe* 
-   when an object is too large to be encoded with a single call,
-   each set of chunks created by a call to the encoding function is
-   called a stripe.
-
-*shard|strip*
-   an ordered sequence of chunks of the same rank from the same
-   object.  For a given placement group, each OSD contains shards of
-   the same rank. When dealing with objects that are encoded with a
-   single operation, *chunk* is sometime used instead of *shard*
-   because the shard is made of a single chunk. The *chunks* in a
-   *shard* are ordered according to the rank of the stripe they belong
-   to.
-
-*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.
-
-*N*
-   the number of data *chunks* plus the number of coding *chunks*, 
-   i.e. *K+M*.
-
-*rate*
-   the proportion of the *chunks* that contains useful information, i.e. *K/N*.
-   For instance, for *K* = 9 and *M* = 3 (i.e. *K+M* = *N* = 12) the rate is 
-   *K* = 9 / *N* = 12 = 0.75, i.e. 75% of the chunks contain useful information.
-
-The definitions are illustrated as follows (PG stands for placement group):
-::
- 
-                 OSD 40                       OSD 33
-       +-------------------------+ +-------------------------+
-       |      shard 0 - PG 10    | |      shard 1 - PG 10    |
-       |+------ object O -------+| |+------ object O -------+|
-       ||+---------------------+|| ||+---------------------+||
- stripe|||    chunk  0         ||| |||    chunk  1         ||| ...
-   0   |||    stripe 0         ||| |||    stripe 0         ||| 
-       ||+---------------------+|| ||+---------------------+||
-       ||+---------------------+|| ||+---------------------+||
- stripe|||    chunk  0         ||| |||    chunk  1         ||| ...
-   1   |||    stripe 1         ||| |||    stripe 1         |||
-       ||+---------------------+|| ||+---------------------+||
-       ||+---------------------+|| ||+---------------------+||
- stripe|||    chunk  0         ||| |||    chunk  1         ||| ...
-   2   |||    stripe 2         ||| |||    stripe 2         |||
-       ||+---------------------+|| ||+---------------------+||
-       |+-----------------------+| |+-----------------------+|
-       |         ...             | |         ...             |
-       +-------------------------+ +-------------------------+
-
-Table of content
-----------------
-
-.. toctree::
-   :maxdepth: 1
-
-   Developer notes <erasure_coding/developer_notes>
-   Jerasure plugin <erasure_coding/jerasure>
-   High level design document <erasure_coding/ecbackend>