mirror of
https://github.com/ceph/ceph
synced 2024-12-22 11:31:55 +00:00
134 lines
3.6 KiB
ReStructuredText
134 lines
3.6 KiB
ReStructuredText
========================
|
|
SHEC erasure code plugin
|
|
========================
|
|
|
|
The *shec* plugin encapsulates the `multiple SHEC
|
|
<https://wiki.ceph.com/Planning/Blueprints/Hammer/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}] \
|
|
[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: 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
|
|
|
|
``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.
|
|
|
|
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 \
|
|
ruleset-failure-domain=host
|
|
$ ceph osd pool create shecpool 256 256 erasure SHECprofile
|