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============================
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
<https://github.com/ceph/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:
.. prompt:: bash $
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 <https://github.com/ceph/ceph/commit/f51d21b53d26d4f27c950cb1ba3f989e713ab325>`_ 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" <https://github.com/ceph/jerasure/blob/master/Manual.pdf>`_ 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.
|
