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November 26th, 2014 00:00

Isilon quorum

Hi,

I'm doing the Isilon Solution Design course, and at the topic of Protection levels and quorum you state that if a cluster remains with 50% or less (quorum lost) then it will enter read only mode.

What happens if  3 node cluster splits into 2 partitions (1 node in the first and 2 nodes in the second)?

Normally the 2 node partition should keep working normally (read/write), But what happens with the 1 node partition? Will it be read only? Or it will be fenced?

Thank you,

Laszlo

46 Posts

November 27th, 2014 20:00

It has to be this way so as to avoid a "split brain" scenario - that is two nodes modifying the metadata of the same file.  This WILL cause data corruption.

An example:  four node cluster split into two, two node clusters.  A mirrored copy of a file exists on each two node clusters. If we allow read write access to that file from both two node clusters, when the cluster again becomes a single cluster, that file is now corrupted when the metadata is combined.  As absurd as this scenario may sound it is possible and would definitely cause data corruption.  However, if we do not allow writes unless there are greater than 50% of nodes patronizing, this scenario is not possible.

Answer provided byBarryBallard

46 Posts

November 26th, 2014 09:00

The two nodes will have read and write access to the data, but the other node will only have read access to the data contained on that node.

Do also note that in this scenario, 3 nodes, the one node split off will only have one of three pieces if data for a protection group so it will not have enough information to provide complete data.

Answer provided by BarryBallard

November 26th, 2014 09:00

Hello,

allowing read-only access to the cluster partition that has lost quorum doesn't sounds OK for me.

Let's suppose that it has enough information to correctly provide the content of a file (a file less than 128k). There is no information whether this file has been modified or not by the partition that still had quorum. So the read only data provided might be outdated. I would prefer a scenario where the cluster partition that has quorum will fence the nodes that are not reachable by the cluster interconnect, or the nodes that have lost quorum will fence themselves.

Kind regards,

Laszlo

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