Dear Cephalopodians,
we use an SSD-only pool to store the metadata of our CephFS.
In the future, we will add a few NVMEs, and in the long-term view, replace the existing SSDs by NVMEs, too.
Thinking this through, I came up with three questions which I do not find answered in the docs (yet).
Currently, we use the following crush-rule:
--------------------------------------------
rule cephfs_metadata {
id 1
type replicated
min_size 1
max_size 10
step take default class ssd
step choose firstn 0 type osd
step emit
}
--------------------------------------------
As you can see, this uses "class ssd".
Now my first question is:
1) Is there a way to specify "take default class (ssd or nvme)"?
Then we could just do this for the migration period, and at some point remove "ssd".
If multi-device-class in a crush rule is not supported yet, the only workaround which comes to my mind right now is to issue:
$ ceph osd crush set-device-class nvme <old_ssd_osd>
for all our old SSD-backed osds, and modify the crush rule to refer to class "nvme" straightaway.
This leads to my second question:
2) Since the OSD IDs do not change, Ceph should not move any data around by changing both the device classes of the OSDs and the device class in the crush rule - correct?
After this operation, adding NVMEs to our cluster should let them automatically join this crush rule, and once all SSDs are replaced with NVMEs,
the workaround is automatically gone.
As long as the SSDs are still there, some tunables might not fit well anymore out of the box, i.e. the "sleep" values for scrub and repair, though.
Here my third question:
3) Are the tunables used for NVME devices the same as for SSD devices?
I do not find any NVME tunables here:
http://docs.ceph.com/docs/master/rados/configuration/osd-config-ref/
Only SSD, HDD and Hybrid are shown.
Cheers,
Oliver
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