Creating Snapclone for clustered disk

[Kishor Raghavani]

Hi All,

We have a LUN presented to our file and print server, the LUN that was presented has run out of space and now we need to make that LUN bigger. The disk group that the LUN resides has also reached its full capacity. So I basically need to copy the LUN from one disk group to another and then enlarge the disk and present it back to the server. From previous experience I know we would have to use a utility like Diskpart so the clustered server can see the bigger disk.

The issue that I am uncertain about is when re-presenting the snapclone disk back to the server and how a MS cluster would react? Also how would I make the LUN see the same drive letter in windows, this is so users don’t have any issues connecting to their shared folders.

Also if someone can point me to a URL explaining how to create a snapclone disk, I would be very grateful. This will be the first time I have used snapclone.

Many Thanks

[Urban Petry]

What Windows OS are you using?
If it's Windows 2000 or Windows 2003 SP0 it's pretty easy, if it's Windows 2003 SP1 or newer it's not as easy.
On Windows 2000/Windows 2003 SP0 shut down all cluster nodes, create the snapclone, unpresent the original vdisk, present the snapclone vdisk with the same LUN as the original vdisk (you might need to use the commandline utility sssu to do this before the snapclone process is finished) and start the cluster nodes again. Then the cluster should see the new snapclone vdisk at the same place (=drive letter) as the original one (this is because the Windows disk signature is used to identify the disk, and this signature is the same on the clone - because it's stored on the first data block of the vdisk).
Beginning with Windows 2003 SP1 the cluster diusk driver does not use the disk signature any more (only as fallback), but uses the "unique ID" read from the SCSI device identification page. On EVA storage systems this "unique ID" is the WWN of the vdisk - and this is obviously different for the snapclone. I don't know whether you can change the WWN of an existing vdisk, but that's what you need to do so that the cluster doesn't recognise any change. The procedure would be the same as above, but before presenting the snapclone vdisk to the cluster nodes you have to make sure that the snapclone vdisk gets the same WWN as the original vdisk had (giving the original vdisk yet another WWN first so that there are no duplicate WWNs).

It should work with a different vdisk WWN also, but Windows might change drive letters etc. so it won't been seemless.

Urban

[Sheldon Smith]

You should also take a look at Microsoft's KB 304736 -- How to extend the partition of a cluster shared disk.

[Kishor Raghavani]

Hi Urban and Sheldon,

Thanks for your replies. Looks like I will have to do some thorough testing in dev lab. Sounds like this could be quite risky. Do you think it might be worth while just doing a normal snapshot?

[Urban Petry]

A snapshot is fine for testing purposes (assuming you use a complete seperate test vdisk and cluster).

I just took a look at the sssu documentation and it is possible to set WWNs for existing vdisks. Only thing to remember is that the vdisk must not be presented to any host when running the command (all on one line):

SET VDISK "\Virtual Disks\archive" WORLD_WIDE_LUN_NAME=6000-1fe1-ff00-0000

Please let us know what you did and how it worked, so that I will know whether the theory proves to be correct ;-)

Urban