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:
- If your CRUSH tunables are set to a value that is not legacy, then reverted back to default value the OSD daemons will not be required to support the feature. Please note that the OSD peering process does require reviewing and comprehending old maps, so you should not run old versions of Ceph if the cluster had previously used a non-legacy CRUSH tunable, even if the latest maps were reverted to legacy default values. It is very important to validate that all OSDs are running the same Ceph version.
The simplest way to adjust the CRUSH tunables is by changing to a known profile. Those are:- legacy: The legacy profile gives the legacy behavior from Argonaut and previous versions
- argonaut: The argonaut profile gives the legacy values that are supported by the original Argonaut release
- bobtail: The values that are supported by the Bobtail release are given by the bobtail profile
- firefly: The values that are supported by Firefly release are given by the firefly profile
- hammer: The values that are supported by the Hammer release are given by the hammer profile
- jewel: The values that are supported by the Jewel release are given by the jewel profile
- optimal: The optimal profile gives the best/optimal values of the current Ceph version
- default: The default values of a new cluster is given by default profile
You can select a profile on a running cluster with the following command:
ceph osd crush tunables {PROFILE}
You can check the current profile on a running cluster with the following command:
ceph osd crush show-tunables
