Recently Dan from CERN showcased a interesting way.
Use upmap to map all PGs to the current OSDs and then change the CRUSH topology.
Then enable the balancer module and have it slowly move the PGs.
Yes, you will rebalance, but it can be done over a very long period with Health OK in between.
Wido Hi All
I'm trying to set the existing pools in a Luminous cluster to use the hdd device-class but without moving data around. If I just create a new rule using the hdd class and set my pools to use that new rule it will cause a huge amount of data movement even though the pgs are all already on HDDs.
There is a thread on ceph-large [1] which appears to have the solution but I can't get my head around what I need to do. I'm not too clear on which IDs I need to swap. Could someone give me some pointers on this please?
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