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         |||
      ||+---------------------+|| ||+---------------------+||
      |+-----------------------+| |+-----------------------+|
      |         ...             | |         ...             |
      +-------------------------+ +-------------------------+