As with replication, Ceph has a concept of a primary OSD, which also exists when using erasure-coded pools. The primary OSD has the responsibility of communicating with the client, calculating the erasure shards, and sending them out to the remaining OSDs in the PG set. This is illustrated in the following diagram:
If an OSD in the set is down, the primary OSD can use the remaining data and erasure shards to reconstruct the data, before sending it back to the client. During read operations, the primary OSD requests all OSDs in the PG set to send their shards. The primary OSD uses data from the data shards ...
