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========================
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:
.. prompt:: bash $
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 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.
: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) = :math:`\frac{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
=============================
.. prompt:: bash $
ceph osd erasure-code-profile set SHECprofile \
plugin=shec \
k=8 m=4 c=3 \
crush-failure-domain=host
ceph osd pool create shecpool erasure SHECprofile
|
