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 ofw=7
is suboptimal becauseblaum_roth
works best whenw+1
is prime. When creating a new erasure-code profile withtechnique=blaum_roth
, setw
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 tow
when using Blaum-Roth coding. (Information about the proper value ofw
when usingblaum_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
orhdd
), 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.