mirror of
https://github.com/ceph/ceph
synced 2024-12-25 21:03:31 +00:00
83 lines
3.2 KiB
ReStructuredText
83 lines
3.2 KiB
ReStructuredText
==============================
|
|
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/pgbackend>
|