Re: I/O hangs when one of three nodes is down

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Hi,


On 06/07/2018 02:52 PM, Фролов Григорий wrote:
?Hello. Could you please help me troubleshoot the issue.

I have 3 nodes in a cluster.
*snipsnap*

root@testk8s2:~# ceph -s
     cluster 0bcc00ec-731a-4734-8d76-599f70f06209
      health HEALTH_ERR
             80 pgs degraded
             80 pgs stuck degraded
             80 pgs stuck unclean
             80 pgs stuck undersized
             80 pgs undersized
             recovery 1075/3225 objects degraded (33.333%)
             mds rank 2 has failed
             mds cluster is degraded
             1 mons down, quorum 1,2 testk8s2,testk8s3
      monmap e1: 3 mons at {testk8s1=10.105.6.116:6789/0,testk8s2=10.105.6.117:6789/0,testk8s3=10.105.6.118:6789/0}
             election epoch 120, quorum 1,2 testk8s2,testk8s3
       fsmap e14084: 2/3/3 up {0=testk8s2=up:active,1=testk8s3=up:active}, 1 failed
      osdmap e9939: 3 osds: 2 up, 2 in; 80 remapped pgs
             flags sortbitwise,require_jewel_osds
       pgmap v17491: 80 pgs, 3 pools, 194 MB data, 1075 objects
             1530 MB used, 16878 MB / 18408 MB avail
             1075/3225 objects degraded (33.333%)
                   80 active+undersized+degraded
I assume all your MDS servers are active MDS. In this setup the filesystem metadata is shared between the hosts. If one of the MDS is not available, the part of the filesystem served by that MDS is not accessible.

You can prevent this kind of lock up by using standby MDS server that will become active as soon as one of the active MDS server fails.

To keep the failover time as low as possible, you can configure a standby MDS to be associated with a running active MDS. You would require one standby MDS for each active MDS, but failover time would be minimal. An unassociated MDS can replace any failed active MDS, but need to load its inode cache before becoming active. This may take some time.

Regards,
Burkhard
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