I strongly recommend running 5 mons. Given how you phrase the above it seems as though you might mean 3 dedicated mon nodes? If so I’d still run 5 mons, add the mon cephadm host label to two of the OSD nodes.
Right now. only one mon is "active" and used all of the time. You would still run 5 mons anyway?
Are you conflating mons and mgrs? Mons form a quorum, you need more than half of them able to reach each other. Yes I would run 5 mons, to survive a double failure. At at least 3 mgrs.
NVME is used only for OS. I was considering offloading WAL+DB to NVME, but that would mean 3-5NVME per OSD/JBOD node and we do not have configuration like that right now.
Nothing stops you from adding those, though for many applications the benefit isn’t worth it.
Do you have your index pool on SSDs? How old are those existing OSDs? If they were deployed in the 6 TB spinner era, they may have the old bluestore_min_alloc_size value baked in. Check `ceph osd metadata`, see if any are not 4KiB. If you have or will have a significant population of small RGW objects, serially redeploying the old OSDs — especially if they’re Filestore — would have advantages.
Index pool is on HDD. HDDs (mostly Seagate) are actually quite old
At 6 TB that was kinda assumed ;)
from 2016 and lately we are seeing a bigger failure rate,
That’s very much to be expected.
I checked bluestore_min_alloc_size and indeed there is a 4K value set for all of them. Even on new ones.
Groovy. You checked with “ceph osd metadata”?
We are replacing failed disks on "same OSD id" with something like:
# destroy failed osd systemctl stop ceph-osd@{id} ceph osd down osd.{id} ceph osd destroy {id} --yes-i-really-mean-it
# prepare new one on same osd id ceph-volume lvm prepare --osd-id {id} --data /dev/sdX ceph-volume lvm list ceph-volume lvm activate {id} {osd fsid}
Is this "a problem"?
Not at all.