On Wed, Nov 15, 2017 at 2:07 PM Bryan Henderson <bryanh@xxxxxxxxxxxxxxxx> wrote:
Some questions about maps and epochs:
I see that I can control the minimum number of osdmap epochs to keep with
"mon min osdmap epoch". Why do I care? Why would I want any but the current
osdmap, and why would the system keep more than my minimum?
Similarly, "mon max pgmap epoch" controls the _maximum_ number of pgmap epochs
to keep around. I believe I need more than the most recent pgmap because I
need to keep previous ones until all PGs that were placed according to that
pgmap have migrated to where the current pgmap says they should be. But do I
need more epochs than that, and what happens if the maximum I set is too low
to cover those necessesary old pgmaps?
Ah, you're a bit backwards here. Despite, the name, the "pgmap" is a reporting structure about how much space PGs use, but it *does not* control where the PGs are placed. That's entirely determined by the OSDMap.
So the cluster needs older OSDMaps to do peering and make sure it gets up-to-date PG data when doing replication/peering/recovery/etc. The PGMaps are only useful for reporting statistics.
I don't think you as a user have any good reason to mess around with these at this point.
-Greg
--
Bryan Henderson San Jose, California
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