Notice

This document is for a development version of Ceph.

SHEC erasure code plugin

The shec plugin encapsulates the multiple 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 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) = \(\frac{4}{4+3}\) = 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

$ 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