+++ /dev/null
-========================
-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 ruleset. 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 ruleset 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
Code Review / stor4nfv.git / blobdiff