Erasure Coded Placement Groups

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 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 definitions are illustrated as follows:

                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

• Developer notes
• Jerasure plugin
• High level design document