On 3/9/21 4:03 PM, Jesper Lykkegaard Karlsen wrote:
Dear Ceph’ers
I am about to upgrade MDS nodes for Cephfs in the Ceph cluster (erasure code 8+3 ) I am administrating.
Since they will get plenty of memory and CPU cores, I was wondering if it would be a good idea to move metadata OSDs (NVMe's currently on OSD nodes together with cephfs_data ODS (HDD)) to the MDS nodes?
Configured as:
4 x MDS with each a metadata OSD and configured with 4 x replication
so each metadata OSD would have a complete copy of metadata.
I know MDS, stores al lot of metadata in RAM, but if metadata OSDs were on MDS nodes, would that not bring down latency?
Anyway, I am just asking for your opinion on this? Pros and cons or even better somebody who actually have tried this?
I doubt you'll gain a lot from this. Data still has to be replicated, so network latency. And reads would come from the primary OSDs from the cephFS metadata pool. So only if you can make all primary OSDs be on the single active MDS you might have gains. But you will have to do manual tuning with upmap to achieve that. I think your money is better spend at buying more NVMe disks and spreading the load than to co-locate that on MDS. If you are planning on multi-active MDS I don't think it would make sense at all. Gr. Stefan