On 15/08/2014 13:24, Wido den Hollander wrote: > On 08/15/2014 12:23 PM, Loic Dachary wrote: >> Hi Erik, >> >> On 15/08/2014 11:54, Erik Logtenberg wrote: >>> Hi, >>> >>> With EC pools in Ceph you are free to choose any K and M parameters you >>> like. The documentation explains what K and M do, so far so good. >>> >>> Now, there are certain combinations of K and M that appear to have more >>> or less the same result. Do any of these combinations have pro's and >>> con's that I should consider and/or are there best practices for >>> choosing the right K/M-parameters? >>> > > Loic might have a better anwser, but I think that the more segments (K) you > have, the heavier recovery. You have to contact more OSDs to reconstruct the > whole object so that involves more disks doing seeks. > > I heard sombody from Fujitsu say that he thought 8/3 was best for most > situations. That wasn't with Ceph though, but with a different system which > implemented Erasure Coding. > >>> For instance, if I choose K = 3 and M = 2, then pg's in this pool will >>> use 5 OSD's and sustain the loss of 2 OSD's. There is 40% overhead in >>> this configuration. >>> >>> Now, if I were to choose K = 6 and M = 4, I would end up with pg's that >>> use 10 OSD's and sustain the loss of 4 OSD's, which is statistically not >>> so much different from the first configuration. Also there is the same >>> 40% overhead. >> >> Although I don't have numbers in mind, I think the odds of loosing two OSD >> simultaneously are a lot smaller than the odds of loosing four OSD >> simultaneously. Or am I misunderstanding you when you write "statistically >> not so much different from the first configuration" ? >> > > Loosing two smaller then loosing four? Is that correct or did you mean it the > other way around?
Right, sorry for the confusion, I meant the other way around :-) > > I'd say that loosing four OSDs simultaneously is less likely to happen then > two simultaneously. > >> Cheers >> >>> One rather obvious difference between the two configurations is that the >>> latter requires a cluster with at least 10 OSD's to make sense. But >>> let's say we have such a cluster, which of the two configurations would >>> be recommended, and why? >>> >>> Thanks, >>> >>> Erik. >>> _______________________________________________ >>> ceph-users mailing list >>> ceph-users@lists.ceph.com >>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >>> >> >> >> >> _______________________________________________ >> ceph-users mailing list >> ceph-users@lists.ceph.com >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >> > > -- Loïc Dachary, Artisan Logiciel Libre
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