Ceph: Designing and Implementing Scalable Storage Systems
by Michael Hackett, Vikhyat Umrao, Karan Singh, Nick Fisk
A few important points
An adjustment to a CRUSH tunable will result in the shift of some PGs between storage nodes. If the Ceph cluster contains a large amount of data already, be prepared that there may be a good amount of PG movement with a CRUSH tunable change.
Monitor and OSD daemons will start and each daemon will require the new enabled CRUSH features of each new connection as they receive the updated maps. Any client that is already connected to the cluster will be grandfathered in; this will lead to unwanted behaviors if the clients (kernel version, Ceph version) do not support the newly enabled features. If you choose to set your CRUSH tunables to optimal, please verify that all Ceph nodes and clients are running the same version: ...
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