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 to construct the requested data, and the erasure shards are discarded. There is a fast read option that can be enabled on erasure pools, which allows the primary OSD to reconstruct the data from erasure shards if they return quicker than data shards. This can help to lower average latency at the cost of a slightly higher CPU usage. The following diagram shows how Ceph reads from an erasure-coded pool:

The following diagram shows how Ceph reads from an erasure pool when one of the data shards is unavailable. Data is reconstructed by reversing the erasure algorithm using the remaining data and erasure shards: