Hi Michael,
Good job! It would be really useful to add in calculations to show the
expected distribution and max deviation from the mean.
I'm dredging this up from an old email I sent out a year ago, but if we
treat this as a "balls into bins" problem ala Raab & Steger:
http://www14.in.tum.de/personen/raab/publ/balls.pdf
I believe we can get a tight bound on the maximally loaded bin where:
- m balls in n bins
- m > n
with the formula:
m/n + sqrt(2m*ln(n)/n)
IE, if we say have 9000 balls spread across 90 bins:
9000/90 + sqrt(2*9000*ln(90)/90) =~ 130
vs 9000/90 = 100 on average
That would allow folks to get a feel for how much deviation they *could*
see given different PG/OSD counts. There are techniques that we can use
to cheat around this like applying new random seeds during pool creation
to throw away particularly bad pool topologies. Unfortunately once the
topology changes you are bound by random variation again. Changing OSD
weight might help, but with multiple pools the skew may be right for one
pool but wrong for another.
On 01/07/2015 04:08 PM, Michael J. Kidd wrote:
Hello all,
Just a quick heads up that we now have a PG calculator to help
determine the proper PG per pool numbers to achieve a target PG per OSD
ratio.
http://ceph.com/pgcalc
Please check it out! Happy to answer any questions, and always welcome
any feedback on the tool / verbiage, etc...
As an aside, we're also working to update the documentation to reflect
the best practices. See Ceph.com tracker for this at:
http://tracker.ceph.com/issues/9867
Thanks!
Michael J. Kidd
Sr. Storage Consultant
Inktank Professional Services
- by Red Hat
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