Virtual system drive is split between separate LUNs

Question

My hardware VMWare guy told me that a Win2008R2 server I have has a D drive that is split between two separate LUNs. He could not tell me if that's a good thing or bad just that it's not standard practice for him.

Would you please explain the benefits or drawbacks of this setup?

Thanks

EDIT Some additional info. What happened was I had D drive already allocated. Then I asked for more. They said there's no more space on whatever LUN my D drive is on so the option they gave me was that part of the D drive will be on one LUN and other part will be on another LUN. Hope that helps

Answer 1

I'm not familiar with a method of splitting a single guest disk across several datastores. However, your engineer may be referring to either:

• Splitting the VM's storage so that seperate volumes (C, D) are on seperate LUNs. So you may have the VMDK for C sat on LUN 1, and the VMDK for D sat on LUN 2
• A copy or migration operation on the VM that has not cleaned up the VMDK on it's previous LUN. So your VM is actively using the disks on one LUN, but there's old unused copies of those VMDK disk files sat on another LUN, too. You can inspect the 'edit settings' screen, or vmx file for the VM to verify which VMDKs are actually being used.
• VMFS Extents.

Answer 2

It is not clear exactly what you mean by having your drive split up, but I don't think it could have any benefits. One thing to note though, is if you use storage vmotion to move the files, all files will be migrated at the same time to a single datastore.

Answer 3

Another way that I can think of that might cause this is if the D drive had another drive mounted as a folder on the D drive. You could verify this by looking at the settings of the Vm and see if there are 3 drives listed.

Answer 4

SAN backed storage can combine space from separate RAID sets in a number of ways. It's possible to extend a LUN by either concatenating or striping it with capacity from another LUN that is not in the same RAID set. The SAN abstracts this so that your host system just sees a bigger block storage device. It's not all that unusual, but if done poorly (say by simply concatenating capacity from two LUNs with very different performance characteristics) it might cause undesirable performance variances from the host OS perspective.

It's also possible to do this in VMware using Extents which is probably what is going on here - these are used to combine multiple LUNS into a single VMware VMFS datastore. It would be a standard practice but it's not necessarily a bad idea. The same caution applies to this as for the earlier example - if the two LUNs differ significantly in their performance then there might be performance variances across the datastore (and within your D-Drive) that are undesirable. There is also a slightly elevated failure risk - if either LUN fails then you lose the whole lot but if all the storage comes from the same SAN then that's not particularly serious.