Multiple disks per server.

["Mickaël Canévet" <canevet@embl.fr>]

Hi,

I'm testing ceph on 4 old servers.

As there is more then one disk per server available for data (2 with 6 
disks and 2 with 10 disks for a total of 32 disks over 4 nodes), I was 
wondering how to define OSDs.

I have choice between one OSD per disk (32 OSDs on the cluster) or one 
OSD per server with one btrfs filesystem over all disks of the server (4 
OSDs on the cluster). Which one is the best solution ?

In the first case, if I lose one disk, I lose only a small part of 
available space. In the other case, if I lose one disk, I lose the whole 
server (as btrfs filesystem is in stripping) much more space.

On the other hand, if I lose the whole server in the first case, I can 
lose all replicates of a data because they may be on two different OSD 
on the same server.

Is there a way to define OSD groups so that we can be sure that 2 
replicates are not on OSDs of the same group (could be usefull for 
multiple OSDs per server, but also mutliple server per computing room - 
if I lose one whole room, a lot of server, I will be sure that I have 
not lost every replicates).

Thanks a lot.
Mickaël
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