Hi,

We're migrating from a Jewel / filestore based cephfs archicture to a
Luminous / buestore based one.

One MUST HAVE is multiple Active MDS daemons. I'm still lacking knowledge
of how it actually works.
After reading the docs and ML we learned that they work by sort of dividing
the responsibilities, each with his own and only directory subtree. (please
correct me if I'm wrong).

Question 1: I'd like to know if it is viable to have 4 MDS daemons, being 3
Active and 1 Standby (or Standby-Replay if that's still possible with
multi-mds).

Basically, what we have is 2 subtrees used by dovecot: INDEX and MAIL.
Their tree is almost identical but INDEX stores all dovecot metadata with
heavy IO going on and MAIL stores actual email files, with much more writes
than reads.

I don't know by now which one could bottleneck the MDS servers most so I
wonder if I can take metrics on MDS usage per pool when it's deployed.
Question 2: If the metadata workloads are very different I wonder if I can
isolate them, like pinning MDS servers X and Y to one of the directories.

Cache Tier is deprecated so,
Question 3: how can I think of a read cache mechanism in Luminous with
bluestore, mainly to keep newly created files (emails that just arrived and
will probably be fetched by the user in a few seconds via IMAP/POP3).

Regards,

Webert Lima
DevOps Engineer at MAV Tecnologia
*Belo Horizonte - Brasil*
*IRC NICK - WebertRLZ*
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