[ceph-users] CRUSH - adding device class to existing rule without causing complete rebalance

Patrick Fruh pf at qloc.de
Mon Nov 13 05:56:39 PST 2017

Hi everyone,

I only have a single rule in my crushmap and only OSDs classed as hdd (after the luminous update):

rule replicated_ruleset {
        id 0
        type replicated
        min_size 1
        max_size 10
        step take default
        step chooseleaf firstn 0 type host
        step emit

Since luminous added device classes, I tried updating the rule to

                step take default class hdd

so I could later add a rule for ssds without the existing pools trying to balance on those.
Since all existing OSDs are classed as hdd I thought nothing should change with this rule change (since the OSDs to be used are the same before and after the change), however after inserting the new crushmap, the whole cluster started rebalancing, after which I quickly reinserted the old crushmap.

So, is there any way to limit my existing pools to HDDs without causing a complete rebalance (at least that's what it looked like)?

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