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I tested this out by setting norebalance and norecover, moving the host buckets under the rack buckets (all of them), and then unsetting. Ceph starts melting down with escalating slow requests, even with backfill and recovery parameters set to throttle. I moved
the host buckets back to the default root bucket, and things mostly came right, but I still had some inactive / unknown pgs that I had to restart some OSDs to get back to health_ok.
I’m sure there’s a way you can tune things or fade in crush weights or something, but I’m happy just moving one at a time.
Our environment has 224 OSDs on 14 hosts, btw.
Cheers,
Sean M
We’re adding in a CRUSH hierarchy retrospectively in preparation for a big expansion. Previously we only had host and osd buckets, and now we’ve added in rack buckets.
I’ve got sensible settings to limit rebalancing set, at least what has worked in the past:
osd_max_backfills = 1
osd_recovery_threads = 1
osd_recovery_priority = 5
osd_client_op_priority = 63
osd_recovery_max_active = 3
I thought it would save a lot of unnecessary data movement if I move the existing host buckets to the new rack buckets all at once, rather than host-by-host. As long as recovery is throttled correctly, it shouldn’t matter how many objects are misplaced, the
thinking goes.
1) Is doing all at once advisable, or am I putting myself at a much greater risk if I do have failures during the rebalance (which could take quite a while)?
2) My failure domain is currently set at the host level. If I want to change the failure domain to ‘rack’, when should I best change this (e.g. after the rebalancing finishes for moving the hosts to the racks)?
v12.2.2 if it makes a difference.
Cheers,
Sean M
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