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