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Diffstat (limited to 'doc/dev/osd_internals/erasure_coding.rst')
| -rw-r--r-- | doc/dev/osd_internals/erasure_coding.rst | 26 |
1 files changed, 18 insertions, 8 deletions
diff --git a/doc/dev/osd_internals/erasure_coding.rst b/doc/dev/osd_internals/erasure_coding.rst index cc1efe4b4bf..0586c46c3bb 100644 --- a/doc/dev/osd_internals/erasure_coding.rst +++ b/doc/dev/osd_internals/erasure_coding.rst @@ -3,8 +3,8 @@ 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 +created in July 2013. It is included in Ceph even before erasure coded +pools are 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 @@ -14,8 +14,14 @@ Glossary -------- *chunk* - when the encoding function is called, it returns chunks of the - same size. + when the encoding function is called, it returns chunks of the same + size. Data chunks which can be concated 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, @@ -23,9 +29,13 @@ Glossary called a stripe. *shard|strip* - the file that holds all chunks of a same rank for a given object. + 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. -Example: +The definitions are illustrated as follows: :: OSD 40 OSD 33 @@ -53,6 +63,6 @@ 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> + Jerasure plugin <erasure_coding/jerasure> + High level design document <erasure_coding/pgbackend> |