Erasure Coded Placement Groups

Glossary

chunk: When the encoding function is called, it returns chunks of the same size as each other. There are two kinds of chunks: (1) data chunks, which can be concatenated to reconstruct the original object, and (2) coding chunks, which can be used to rebuild a lost chunk.
chunk rank: The index of a chunk, as determined by the encoding function. The rank of the first chunk is 0, the rank of the second chunk is 1, and so on.
K: The number of data chunks into which an object is divided. For example, if K = 2, then a 10KB object is divided into two objects of 5KB each.
M: The number of coding chunks computed by the encoding function. M is equal to the number of OSDs that can be missing from the cluster without the cluster suffering data loss. For example, if there are two coding chunks, then two OSDs can be missing without data loss.
N: The number of data chunks plus the number of coding chunks: that is, K + M.
rate: The proportion of the total chunks containing useful information: that is, K divided by N. For example, suppose that K = 9 and M = 3. This would mean that N = 12 (because K + M = 9 + 3). Therefore, the rate (K / N) would be 9 / 12 = 0.75. In other words, 75% of the chunks would contain useful information.
shard (also called 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. In the special case in which an object is encoded with only one call to the encoding function, the term chunk may be 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 (see stripe below) they belong to.
stripe: If an object is so large that encoding it requires more than one call to the encoding function, each of these calls creates a set of chunks called a stripe.

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

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