[ceph-users] Why keep old epochs?

Bryan Henderson bryanh at giraffe-data.com
Tue Nov 14 18:59:31 PST 2017

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?

Bryan Henderson                                   San Jose, California

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