Re: Many OSDs on one node and replica distribution

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You'll have to check your crush rule to determine that.

ceph osd getcrushmap -o crushmap
crushtool -d crushmap -o crushmap.txt
vi crushmap.txt


check the rules near the end of that file.  Rule 0 shows placement by host, and rule 1 shows placemeny by osd.  You can add another rule to your config and then change any pool to use the other rule.


# rules
rule sandbox_host {
        ruleset 0
        type replicated
        min_size 2
        max_size 10
        step take sandbox
        step chooseleaf firstn 0 type host
        step emit
}
rule sandbox_osd {
        ruleset 1
        type replicated
        min_size 1
        max_size 10
        step take sandbox
        step chooseleaf firstn 0 type osd
        step emit
}


To put new rule into use:

crushtool -c crushmap.txt -o crushmap
ceph osd setcrushmap -i crushmap

example of how to change "rbd" pool to "rule 0"

ceph osd pool set rbd crush_ruleset 0

Hope this helps!




-----Original Message-----
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Rene Hadler
Sent: Wednesday, November 26, 2014 7:15 AM
To: ceph-users@xxxxxxxxxxxxxx
Subject:  Many OSDs on one node and replica distribution

Hi dear list,



i have a question about distribution of replicas on hosts with multiple

OSDs.



For example this configuration:

4x nodes

each node has 4 OSDs

replica set to 3



When i save now an object to the pool, how it is replicated? Is there a

chance that the original object and the 2 replicas are stored on the

same node? 



I cant believe that but it was never clear for me. If yes and the node

fails this means the object is destroyed. Or should i set the replica

count to 5 to be sure?



Thanks.



-- 

Mit freundlichen Grüßen / Best regards

Rene Hadler





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