Jerasure erasure code plugin

The jerasure plugin is the most generic and flexible plugin, it is also the default for Ceph erasure coded pools.

The jerasure plugin encapsulates the Jerasure library. It is recommended to read the jerasure documentation to understand the parameters. Note that the jerasure.org web site as of 2023 may no longer be connected to the original project or legitimate.

Create a jerasure profile

To create a new jerasure erasure code profile:

ceph osd erasure-code-profile set {name} \
  plugin=jerasure \
  k={data-chunks} \
  m={coding-chunks} \
  technique={reed_sol_van|reed_sol_r6_op|cauchy_orig|cauchy_good|liberation|blaum_roth|liber8tion} \
  [crush-root={root}] \
  [crush-failure-domain={bucket-type}] \
  [crush-device-class={device-class}] \
  [directory={directory}] \
  [--force]

Where:

k={data chunks}

Description: Each object is split in data-chunks parts, each stored on a different OSD.
Type: Integer
Required: Yes.
Example: 4

m={coding-chunks}

Description: Compute coding chunks for each object and store them on different OSDs. The number of coding chunks is also the number of OSDs that can be down without losing data.
Type: Integer
Required: Yes.
Example: 2

technique={reed_sol_van|reed_sol_r6_op|cauchy_orig|cauchy_good|liberation|blaum_roth|liber8tion}

Description: The more flexible technique is reed_sol_van : it is enough to set k and m. The cauchy_good technique can be faster but you need to chose the packetsize carefully. All of reed_sol_r6_op, liberation, blaum_roth, liber8tion are RAID6 equivalents in the sense that they can only be configured with m=2.

Note

When using blaum_roth coding, the default word size of w=7 is suboptimal because blaum_roth works best when w+1 is prime. When creating a new erasure-code profile with technique=blaum_roth, set w to a number that is one integer less than a prime number (for example, 6). See Loic Dachary’s commit f51d21b to ceph/ceph for information about why this default cannot be changed easily in the source code, and see the second bullet point on page 29 of Plank and Greenan’s “Jerasure: A Library in C Facilitating Erasure Coding for Storage Applications” for an unequivocal statement of the restriction that applies to w when using Blaum-Roth coding. (Information about the proper value of w when using blaum_roth coding was provided to the Ceph upstream in September of 2024 by Benjamin Mare.)
Type: String
Required: No.
Default: reed_sol_van

packetsize={bytes}

Description: The encoding will be done on packets of bytes size at a time. Choosing the right packet size is difficult. The jerasure documentation contains extensive information on this topic.
Type: Integer
Required: No.
Default: 2048

crush-root={root}

Description: The name of the crush bucket used for the first step of the CRUSH rule. For instance step take default.
Type: String
Required: No.
Default: default

crush-failure-domain={bucket-type}

Description: Ensure that no two chunks are in a bucket with the same failure domain. For instance, if the failure domain is host no two chunks will be stored on the same host. It is used to create a CRUSH rule step such as step chooseleaf host.
Type: String
Required: No.
Default: host

crush-device-class={device-class}

Description: Restrict placement to devices of a specific class (e.g., ssd or hdd), using the crush device class names in the CRUSH map.
Type: String
Required: No.

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

--force

Description: Override an existing profile by the same name.
Type: String
Required: No.

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