documentation: jerasure plugin is sub section of erasure code profile

Signed-off-by: Loic Dachary <loic-201408@dachary.org>

diff --git a/doc/rados/operations/erasure-code-jerasure.rst b/doc/rados/operations/erasure-code-jerasure.rst
new file mode 100644 (file)
index 0000000..bd2917e
--- /dev/null
+++ b/doc/rados/operations/erasure-code-jerasure.rst
@@ -0,0 +1,109 @@
+============================
+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://bitbucket.org/jimplank/jerasure/>`_ library. It is
+recommended to read the *jerasure* documentation to get a better
+understanding of the parameters.
+
+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} \
+             [ruleset-root={root}] \
+             [ruleset-failure-domain={bucket-type}] \
+             [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*. 
+
+:Type: String
+:Required: No.
+:Default: reed_sol_van
+
+``packetsize={bytes}``
+
+:Description: The encoding will be done on packets of *bytes* size at
+              a time. Chosing the right packet size is difficult. The
+              *jerasure* documentation contains extensive information
+              on this topic.
+
+:Type: Integer
+:Required: No.
+:Default: 2048
+
+``ruleset-root={root}``
+
+:Description: The name of the crush bucket used for the first step of
+              the ruleset. For intance **step take default**.
+
+:Type: String
+:Required: No.
+:Default: default
+
+``ruleset-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 ruleset step such as **step
+              chooseleaf host**.
+
+:Type: String
+:Required: No.
+:Default: host
+
+``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.
+