Hi to all!
I'm searching the wisdom of the community on how to drain/fill an OSD
server (like for removal, bulk disk replacing, filestore to bluestore
conversion...) without impacting on the performance of the cluster.
We are using the 'ceph-gentle-reweight' script, which is running fine,
but we are interested to know if there is a newer approach for this job.
Searching the web I found a pdf of the talk of Dan van der Ster about
"Mastering Ceph Operations" (NYC Ceph Day 2024) in which he talks about
using the 'balancer' in 'upmap' mode (and a "magic" script) to move the
PGs from/to the OSDs in the background.
So the balancer upmap approach can be used to drain a node following
this procedure:
- set balancer tunables (like 'target_max_misplaced_ratio')
- activate the balancer in upmap mode (if not already active)
- 'ceph osd crush reweight-subtree' to 0.1 for the host we wish to drain
- run the magic script 'upmap-remapped.py'
- 'ceph osd crush reweight-subtree' to 0 for the host we wish to drain
- do stuff on the node (remove, reinstall etc etc).
But what is the implication for the whole cluster, how this background
job affects its performance?
I assume that 'target_max_misplaced_ratio' is a key parameter to avoid
large data movement in a given time, there is more to look into?
And there are other approaches for the job?
Thanks a lot!
Iztok
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