Erasure code profiles

Erasure code is defined by a profile and is used when creating an erasure coded pool and the associated CRUSH rule.

The default erasure code profile (which is created when the Ceph cluster is initialized) provides the same level of redundancy as two copies but requires 25% less disk space. It is described as a profile with k=2 and m=1, meaning the information is spread over three OSD (k+m == 3) and one of them can be lost.

To improve redundancy without increasing raw storage requirements, a new profile can be created. 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. The object is first divided in 10 chunks (if the object is 10MB, each chunk is 1MB) and 4 coding chunks are computed, 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 break at the same time.

osd erasure-code-profile set

To create a new erasure code profile:

ceph osd erasure-code-profile set {name} \
     [{directory=directory}] \
     [{plugin=plugin}] \
     [{stripe_unit=stripe_unit}] \
     [{key=value} ...] \
     [--force]

Where:

{directory=directory}

Description:Set the directory name from which the erasure code plugin is loaded.
Type:String
Required:No.
Default:/usr/lib/ceph/erasure-code

{plugin=plugin}

Description:Use the erasure code plugin to compute coding chunks and recover missing chunks. See the list of available plugins for more information.
Type:String
Required:No.
Default:jerasure

{stripe_unit=stripe_unit}

Description:The amount of data in a data chunk, per stripe. For example, a profile with 2 data chunks and stripe_unit=4K would put the range 0-4K in chunk 0, 4K-8K in chunk 1, then 8K-12K in chunk 0 again. This should be a multiple of 4K for best performance. The default value is taken from the monitor config option osd_pool_erasure_code_stripe_unit when a pool is created. The stripe_width of a pool using this profile will be the number of data chunks multiplied by this stripe_unit.
Type:String
Required:No.

{key=value}

Description:The semantic of the remaining key/value pairs is defined by the erasure code plugin.
Type:String
Required:No.

--force

Description:Override an existing profile by the same name, and allow setting a non-4K-aligned stripe_unit.
Type:String
Required:No.

osd erasure-code-profile rm

To remove an erasure code profile:

ceph osd erasure-code-profile rm {name}

If the profile is referenced by a pool, the deletion will fail.

osd erasure-code-profile get

To display an erasure code profile:

ceph osd erasure-code-profile get {name}

osd erasure-code-profile ls

To list the names of all erasure code profiles:

ceph osd erasure-code-profile ls