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
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