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83 lines
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==============================
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Erasure Coded Placement Groups
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==============================
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Glossary
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--------
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*chunk*
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when the encoding function is called, it returns chunks of the same
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size. Data chunks which can be concatenated to reconstruct the original
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object and coding chunks which can be used to rebuild a lost chunk.
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*chunk rank*
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the index of a chunk when returned by the encoding function. The
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rank of the first chunk is 0, the rank of the second chunk is 1
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etc.
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*stripe*
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when an object is too large to be encoded with a single call,
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each set of chunks created by a call to the encoding function is
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called a stripe.
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*shard|strip*
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an ordered sequence of chunks of the same rank from the same
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object. For a given placement group, each OSD contains shards of
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the same rank. When dealing with objects that are encoded with a
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single operation, *chunk* is sometime used instead of *shard*
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because the shard is made of a single chunk. The *chunks* in a
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*shard* are ordered according to the rank of the stripe they belong
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to.
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*K*
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the number of data *chunks*, i.e. the number of *chunks* in which the
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original object is divided. For instance if *K* = 2 a 10KB object
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will be divided into *K* objects of 5KB each.
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*M*
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the number of coding *chunks*, i.e. the number of additional *chunks*
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computed by the encoding functions. If there are 2 coding *chunks*,
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it means 2 OSDs can be out without losing data.
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*N*
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the number of data *chunks* plus the number of coding *chunks*,
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i.e. *K+M*.
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*rate*
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the proportion of the *chunks* that contains useful information, i.e. *K/N*.
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For instance, for *K* = 9 and *M* = 3 (i.e. *K+M* = *N* = 12) the rate is
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*K* = 9 / *N* = 12 = 0.75, i.e. 75% of the chunks contain useful information.
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The definitions are illustrated as follows (PG stands for placement group):
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::
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OSD 40 OSD 33
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+-------------------------+ +-------------------------+
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| shard 0 - PG 10 | | shard 1 - PG 10 |
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|+------ object O -------+| |+------ object O -------+|
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||+---------------------+|| ||+---------------------+||
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stripe||| chunk 0 ||| ||| chunk 1 ||| ...
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0 ||| stripe 0 ||| ||| stripe 0 |||
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||+---------------------+|| ||+---------------------+||
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||+---------------------+|| ||+---------------------+||
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stripe||| chunk 0 ||| ||| chunk 1 ||| ...
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1 ||| stripe 1 ||| ||| stripe 1 |||
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||+---------------------+|| ||+---------------------+||
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||+---------------------+|| ||+---------------------+||
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stripe||| chunk 0 ||| ||| chunk 1 ||| ...
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2 ||| stripe 2 ||| ||| stripe 2 |||
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||+---------------------+|| ||+---------------------+||
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|+-----------------------+| |+-----------------------+|
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| ... | | ... |
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+-------------------------+ +-------------------------+
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Table of content
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----------------
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.. toctree::
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:maxdepth: 1
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Developer notes <erasure_coding/developer_notes>
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Jerasure plugin <erasure_coding/jerasure>
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High level design document <erasure_coding/ecbackend>
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