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.
Current op-version can be queried as below:
[root@~]#gluster volume get <volume name> cluster.op-version
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.