Chapter 34. Erasure Code Profiles

Ceph defines an erasure-coded pool with a profile. Ceph uses a profile when creating an erasure-coded pool and the associated crush ruleset.

Ceph creates a default erasure code profile when initializing a cluster, and it provides the same level of redundancy as two copies in a replicated pool, but uses 25% less storage capacity. The default profiles defines k=2 and m=1, meaning Ceph will spread the object data over three OSDs (k+m == 3) and Ceph can lose one of those OSDs without losing data.

You can create a new profile to improve redundancy without increasing raw storage requirements. For instance, a profile with k=10 and m=4 can sustain the loss of four (m=4) OSDs by distributing an object on fourteen (k+m=14) OSDs. Ceph divides the object into 10 chunks (if the object is 10MB, each chunk is 1MB) and computes 4 coding chunks for recovery (each coding chunk has the same size as the data chunk, i.e. 1MB). The raw space overhead is only 40% and the object will not be lost even if four OSDs fail simultaneously.

ceph osd erasure-code-profile set <name> \
         [<directory=directory>] \
         [<plugin=plugin>] \
         [<key=value> ...] \
         [--force]

ceph osd erasure-code-profile rm <name>

ceph osd erasure-code-profile get <name>

ceph osd erasure-code-profile ls