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erasure-code: add definitions to the glossary

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and fix a typo

Signed-off-by: Loic Dachary <loic@dachary.org>
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Loic Dachary 2014-06-30 10:19:23 +02:00
parent 62027ec9f4
commit 1dfe913af2
1 changed files with 29 additions and 7 deletions
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36
doc/dev/osd_internals/erasure_coding.rst
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@ -7,7 +7,7 @@ Glossary
*chunk*
when the encoding function is called, it returns chunks of the same
size. Data chunks which can be concated to reconstruct the original
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*
@ -25,9 +25,30 @@ Glossary
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.
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.
The definitions are illustrated as follows:
*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 loosing 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
@ -36,15 +57,16 @@ The definitions are illustrated as follows:
|+------ object O -------+| |+------ object O -------+|
||+---------------------+|| ||+---------------------+||
stripe||| chunk 0 ||| ||| chunk 1 ||| ...
0 ||| [0,+N) ||| ||| [0,+N) |||
0 ||| stripe 0 ||| ||| stripe 0 |||
||+---------------------+|| ||+---------------------+||
||+---------------------+|| ||+---------------------+||
stripe||| chunk 0 ||| ||| chunk 1 ||| ...
1 ||| [N,+N) ||| ||| [N,+N) |||
1 ||| stripe 1 ||| ||| stripe 1 |||
||+---------------------+|| ||+---------------------+||
||+---------------------+|| ||+---------------------+||
stripe||| chunk 0 [N*2,+len) ||| ||| chunk 1 [N*2,+len) ||| ...
2 ||+---------------------+|| ||+---------------------+||
stripe||| chunk 0 ||| ||| chunk 1 ||| ...
2 ||| stripe 2 ||| ||| stripe 2 |||
||+---------------------+|| ||+---------------------+||
|+-----------------------+| |+-----------------------+|
| ... | | ... |
+-------------------------+ +-------------------------+