========================
SHEC erasure code plugin
========================

The *shec* plugin encapsulates the `multiple SHEC
<http://tracker.ceph.com/projects/ceph/wiki/Shingled_Erasure_Code_(SHEC)>`_
library. It allows ceph to recover data more efficiently than Reed Solomon codes.

Create an SHEC profile
======================

To create a new *shec* erasure code profile::

        ceph osd erasure-code-profile set {name} \
             plugin=shec \
             [k={data-chunks}] \
             [m={coding-chunks}] \
             [c={durability-estimator}] \
             [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: No.
:Default: 4

``m={coding-chunks}``

:Description: Compute **coding-chunks** for each object and store them on
              different OSDs. The number of **coding-chunks** does not necessarily
              equal the number of OSDs that can be down without losing data.

:Type: Integer
:Required: No.
:Default: 3

``c={durability-estimator}``

:Description: The number of parity chunks each of which includes each data chunk in its
              calculation range. The number is used as a **durability estimator**.
              For instance, if c=2, 2 OSDs can be down without losing data.

:Type: Integer
:Required: No.
:Default: 2

``crush-root={root}``

:Description: The name of the crush bucket used for the first step of
              the CRUSH rule. For intance **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.
:Default:

``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.

Brief description of SHEC's layouts
===================================

Space Efficiency
----------------

Space efficiency is a ratio of data chunks to all ones in a object and
represented as k/(k+m).
In order to improve space efficiency, you should increase k or decrease m.

::

        space efficiency of SHEC(4,3,2) = 4/(4+3) = 0.57
        SHEC(5,3,2) or SHEC(4,2,2) improves SHEC(4,3,2)'s space efficiency

Durability
----------

The third parameter of SHEC (=c) is a durability estimator, which approximates
the number of OSDs that can be down without losing data.

``durability estimator of SHEC(4,3,2) = 2``

Recovery Efficiency
-------------------

Describing calculation of recovery efficiency is beyond the scope of this document,
but at least increasing m without increasing c achieves improvement of recovery efficiency.
(However, we must pay attention to the sacrifice of space efficiency in this case.)

``SHEC(4,2,2) -> SHEC(4,3,2) : achieves improvement of recovery efficiency``

Erasure code profile examples
=============================

::

        $ ceph osd erasure-code-profile set SHECprofile \
             plugin=shec \
             k=8 m=4 c=3 \
             crush-failure-domain=host
        $ ceph osd pool create shecpool 256 256 erasure SHECprofile
