Pushing Virtual Limits

RDM’s Rock
April 3, 2008, 2:17 am
Filed under: VM

I just wanted to put a quick note up after spending some time working with RDM volumes. Having read all the vmware white papers on the subject for a major client I see that they are not huge fans of it.

There is minimal to no performance increase. It sounds like there is an additional load on the host for the overhead. The volume is less portable than a VMDK.

I have never tried to use convertor to export an image with an RDM volume attached (something to try tomorrow) but I suspect that won’t work.

The list of potential negatives goes on and on but the positive is a simple one. Here is a great use case I hear frequently –

Customer’s application requires a file share and it is full. Their software allows them to utilize 32 file shares which would seemingly be enough for anyone  but they had a stingy SAN admin who gave them storage in 50 and 100GB blocks. With VMDK’s you can jump through hoops and extend the volume and then make adjustments on the VM but with an RDM volume it is exceptionally easy to extend the LUN and then capture that space in the VM.

The other huge value add comes in P2V situations.

Customer is migrating to a virtual solution from a physical installation that utilizes four 2-3 TB LUNs. Robocopying that data is a nightmare! Utilizing RDM you can simply add it to the new VM and your data will appear as if by magic. No migration, no corruption, no problems.

Yet another way the future of computing has been brought to you by VMWare.


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