On 08/15/2014 06:24 AM, 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. Performance is definitely lower with more segments in Ceph. I kind of gravitate toward 4/2 or 6/2, though that's just my own preference. > >>> 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? > > I'd say that loosing four OSDs simultaneously is less likely to happen > then two simultaneously. This is true, though the more disks you spread your objects across, the higher likelihood that any given object will be affected by a lost OSD. The extreme case being that every object is spread across every OSD and losing any given OSD affects all objects. I suppose the severity depends on the relative fraction of your erasure coding parameters relative to the total number of OSDs. I think this is perhaps what Erik was getting at. > >> 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 at lists.ceph.com >>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >>> >> >> >> >> _______________________________________________ >> ceph-users mailing list >> ceph-users at lists.ceph.com >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >> > >
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