###op-version
op-version is the operating version of the Gluster which is running.

op-version was introduced to ensure gluster running with different versions do not end up in a problem and backward compatibility issues can be tackled.

After Gluster upgrade, it is advisable to have op-version updated.

###Updating op-version

Current op-version can be queried as below:

For 3.10 onwards:

    [root@~]#gluster volume get all cluster.op-version

For release < 3.10:

    [root@~]#gluster volume get <VOLNAME> cluster.op-version

To get the maximum possible op-version a cluster can support, the following query can be used (this is available 3.10 release onwards):

    [root@~]#gluster volume get all cluster.max-op-version

For example, if some nodes in a cluster have been upgraded to X and some to X+, then the maximum op-version supported by the cluster is X, and the cluster.op-version can be bumped up to X to support new features.

op-version can be updated as below.
For example, after upgrading to glusterfs-3.7.1, set op-version as:

    [root@~]#gluster volume set all cluster.op-version 30701

Note: 
This is not mandatory, but advisable to have updated op-version if you want to make use of latest features in the updated gluster.

###Client op-version

When trying to set a volume option, it might happen that one or more of the connected clients cannot support the feature being set and might need to be upgraded to the op-version the cluster is currently running on.

To check op-version information for the connected clients and find the offending client, the following query can be used for 3.10 release onwards:

    [root@~]#gluster volume status <all|VOLNAME> clients

The respective clients can then be upgraded to the required version.

This information could also be used to make an informed decision while bumping up the op-version of a cluster, so that connected clients can support all the new features provided by the upgraded cluster as well.