CRUSH MAP RULES

CRUSH maps support the notion of ‘CRUSH rules’, which are the rules that determine data placement for a pool. For large clusters, you will likely create many pools where each pool may have its own CRUSH ruleset and rules. The default CRUSH map has a rule for each pool, and one ruleset assigned to each of the default pools.

CRUSH rules define placement and replication strategies or distribution policies that allow you to specify exactly how CRUSH places object replicas. For example, you might create a rule selecting a pair of targets for 2-way mirroring, another rule for selecting three targets in two different data centers for 3-way mirroring, and yet another rule for erasure coding over six storage devices. For a detailed discussion of CRUSH rules, refer to CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data, and more specifically to Section 3.2.

A rule takes the following form:

rule <rulename> {

        ruleset <ruleset>
        type [ replicated | erasure ]
        min_size <min-size>
        max_size <max-size>
        step take <bucket-name> [class <device-class>]
        step [choose|chooseleaf] [firstn|indep] <N> <bucket-type>
        step emit
}

ruleset

type

min_size

max_size

step take <bucket-name> [class <device-class>]

step choose firstn {num} type {bucket-type}

step chooseleaf firstn {num} type {bucket-type}

step emit