Support updated me that there is no safe way to enable VCM bit. If a the vcm state is changed while the hosts are accessing disks then there is a chance that data corruption could occur. For this reason the hosts will need to be temporarily stop i/o on these paths - i.e We need to stop the db engine, umount the fs, stop the VxVM volumes (or whatever is relevant for your OS).
Has anybody enabled VCM bit on the FAs which are connected to Hosts ? Any better recommendations where there is no or minimal disruption to the host ?
i have not done this but i am suspecting that not only will you need to do the steps you already mentioned (stop db, umount fs ..etc) but also be ready to mask devices to appropriate host's WWNs. When VCM bit is not enabled on FA (for Celerra gateway, Mainframe) these systems pretty much see everything that gets mapped to that FA. Do you have open systems connected to FAs that do not have VCM enabled ?
If you have multiple paths to the storage and multi-pathing software (eg EMC PowerPath or equivalent) which is correctly configured and which you have tested and have confidance in - then you could carry out this work on one path at a time without having to bring the hosts down.
We did it on a DMX800 .. When we installed the box, VCM was disabled (since all the storage was connected via switches to a single cluster) .. Later the customer asked to add more hosts on the same ports .. So we mapped the VCM on the ports and enabled the VCM flag just like Marc suggested. Right after activating VCM flag on a port we masked the devices to the host and verified that the paths was again available before moving to the next FA.
It went smoothly and worked fine (sometime it happens)
I enabled the VCM bit this monday morning and all went fine .
This was the plan I followed :
1. Verify the multi-pathing configuration of each affected host. This is to ensure that the host can remain 'up' while the change takes place. There should be at least 1 path to the storage 2. Bring the port offline on the switch which affects the FA port. 3. Bring Offline the FA port on the particular storage array. 4. Enable the VCM bit 5. Online the FA port. 6. Bring the switch port back online. 7. Ensure that the paths have returned.