>>If you have K=2,M=1 you will survive one node failure. If your failure domain is the host (i.e. there never is more than one chunk per node for any given object), it will also survive two disks failures within a given node >>because only one of them will have a chunk. It won't be able to resist the simultaneous failure of two OSDs that belong to two different nodes: that would be the same as having two simultaneous node failure. Ah, ok, it's clear now ! With 1 chunk by node, I finally understand how it's working :) Thanks Loic ----- Mail original ----- De: "Loic Dachary" <loic@xxxxxxxxxxx> À: "aderumier" <aderumier@xxxxxxxxx>, "ceph-users" <ceph-users@xxxxxxxxxxxxxx> Envoyé: Dimanche 1 Février 2015 18:42:51 Objet: Re: erasure code : number of chunks for a small cluster ? Hi Alexandre, On 01/02/2015 18:15, Alexandre DERUMIER wrote: > Hi, > > I'm currently trying to understand how to setup correctly a pool with erasure code > > > https://ceph.com/docs/v0.80/dev/osd_internals/erasure_coding/developer_notes/ > > > My cluster is 3 nodes with 6 osd for each node (18 osd total). > > I want to be able to survive of 2 disk failures, but also a full node failure. If you have K=2,M=1 you will survive one node failure. If your failure domain is the host (i.e. there never is more than one chunk per node for any given object), it will also survive two disks failures within a given node because only one of them will have a chunk. It won't be able to resist the simultaneous failure of two OSDs that belong to two different nodes: that would be the same as having two simultaneous node failure. > > What is the best setup for this ? Does I need M=2 or M=6 ? > > > > > Also, how to determinate the best chunk number ? > > for example, > K = 4 , M=2 > K = 8 , M=2 > K = 16 , M=2 > > you can loose which each config 2 osd, but the more data chunks you have, the less space is used by coding chunks right ? Yes. > Does the number of chunk have performance impact ? (read/write ?) If there are more chunks there is an additional computation overhead but I'm not sure what's the impact. I suspect it's not significant when but never actually measured it. Cheers > > Regards, > > Alexandre > > > > > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > -- Loïc Dachary, Artisan Logiciel Libre _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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