Description of problem:

Consider a cluster with device all HDDs. 

- Initially the ruleset "test" doesn't have a device class associated with it 

rule test {
        id 2
        type replicated
        min_size 1
        max_size 10
        step take default            =========> hdd device class missing
        step chooseleaf firstn 0 type host
        step emit
}


- Edited the crushmap and added the device class

rule test {
        id 2
        type replicated
        min_size 1
        max_size 10
        step take default class hdd  =========> hdd device class present
        step chooseleaf firstn 0 type host
        step emit
}


After injecting the new crush map, we noticed some data movement. Ideally there shouldn't be any data movement as there is no change in the underlying osd disks types.

Is this expected behaviour? If yes,is there any workaround to prevent it as it causes large data movement?


Version-Release number of selected component (if applicable):
3.1

How reproducible:
Always