Hello,
recently I thought about erasure coding and how to set k+m in a useful
way also taking into account the number of hosts available for ceph. Say
I would have this setup:
The cluster has 6 hosts and I want to allow two *hosts* to fail without
loosing data. So I might choose k+m as 4+2 with redundancy at host
level, but isn't this a little unwise?
What would happen if:
1. two disks would fail where both failed disks are not on the same
host? I think ceph would be able to find a PG distributed across all
hosts avoiding the two failed disks, so ceph would be able to repair and
reach a healthy status after a while?
2. Two complete hosts would fail say because of broken power supplies.
In this case ceph would no longer be able to repair the damage because
there are no two more "free" remaining hosts to satisfy the 4+2 rule
(with redundancy on host level). So data would not be lost but the
cluster might stop delivering data and would be unable to repair and
thus would also be unable to become healthy again?
Right or wrong?
Thanks a lot
Rainer
--
Rainer Krienke, Uni Koblenz, Rechenzentrum, A22, Universitaetsstrasse 1
56070 Koblenz, Web: http://www.uni-koblenz.de/~krienke, Tel: +49261287 1312
PGP: http://www.uni-koblenz.de/~krienke/mypgp.html, Fax: +49261287
1001312
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