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==============================
Erasure Coded Placement Groups
==============================
The documentation of the erasure coding implementation in Ceph was
created in July 2013. It is included in Ceph even before erasure
coding is available because it drives a number of architectural
changes. It is meant to be updated to reflect the `progress of these
architectural changes <http://tracker.ceph.com/issues/4929>`_, up to
the point where it becomes a reference of the erasure coding
implementation itself.
Glossary
--------
*chunk*
when the encoding function is called, it returns chunks of the
same size.
*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*
the file that holds all chunks of a same rank for a given object.
Example:
::
OSD 40 OSD 33
+-------------------------+ +-------------------------+
| shard 0 - PG 10 | | shard 1 - PG 10 |
|+------ object O -------+| |+------ object O -------+|
||+---------------------+|| ||+---------------------+||
stripe||| chunk 0 ||| ||| chunk 1 ||| ...
0 ||| [0,+N) ||| ||| [0,+N) |||
||+---------------------+|| ||+---------------------+||
||+---------------------+|| ||+---------------------+||
stripe||| chunk 0 ||| ||| chunk 1 ||| ...
1 ||| [N,+N) ||| ||| [N,+N) |||
||+---------------------+|| ||+---------------------+||
||+---------------------+|| ||+---------------------+||
stripe||| chunk 0 [N*2,+len) ||| ||| chunk 1 [N*2,+len) ||| ...
2 ||+---------------------+|| ||+---------------------+||
|+-----------------------+| |+-----------------------+|
| ... | | ... |
+-------------------------+ +-------------------------+
Table of content
----------------
.. toctree::
:maxdepth: 1
High level design document <erasure_coding/pgbackend>
Developer notes <erasure_coding/developer_notes>
Draft PGBackend.h header <erasure_coding/PGBackend-h>
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