On Tue, 14 Jun 2016, kefu chai wrote: > On Thu, Mar 3, 2016 at 3:15 AM, Sage Weil <sage@xxxxxxxxxxxx> wrote: > > Latest branch: > > > > https://github.com/liewegas/ceph/commits/wip-reweight > > > > I made several changes: > > > > - new command, 'osd utilization' > > > > avg 51 > > stddev 0.707107 (expected baseline 6.68019) > > Sage, sorry for bringing this up again. I don't quite understand what > the "expected baseline" is for. per the code, > > float edev = sqrt(avg_pg * (1.0 - (1.0 / (double)num_up_in))); > > so it sounds sound sort of std dev, but the unit of std dev here is > the "number of pg", while edev's unit is > num_pg^{1/2}, so it's not comparable with avg_pg or the > {base,new}_stddev. so could you shed some light on how we shall use > this number as a reference? > and what's the meaning of this number? (Re-reading 4.1 from the CRUSH paper... I always forget the math) The average osd utilization for the binomial placement process is mu = n*p (p=1/num_osds, n=num pgs) = num_pgs/num_osds, and the standard deviation is sqrt(n*p*(1-p)) = sqrt(avg_pgs*(1-p)) = sqrt(avg_pgs*(1-(1/num_osds))). The units for standard deviation are weird, yep. In any case, the baseline is there to tell you what the statistically expected std dev is so you can tell if something is awry... sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
