RDM versus vmfs lun

[Bluemoon404]

I am currently restructuring our physical server and SAN environment. I am unsure what the advantages are, of going RDM over vmdk.

Example

I have a physical SQL server which has it’s database stored on a RAID 5 group on our SAN and it stores it’s log files on a RAID 1 group also on the SAN, all very straight forward stuff.

My question is:

What is the difference between creating a new datastore on my SAN with a RAID 5 config and only having 1 vmdk which is the D drive of my SQL server holding its databases. I would then create a new RAID 1 datastore and hold the SQL servers log files on it. This setup would use the same amount of disks and only one server would be accessing the LUN.

I can’t see any differences in this scenario between RDM and 1 VMDK on a LUN.

Any experiences, advice is much appreciated.

Paul

[AndreTheGiant · Accepted solution]

RDM is needed for guest clusting and to speed up a P2V when the data are already in the same SAN.

In other cases a vmdk is a better choice.

See also:

Andre

[mittim12]

I agree with Andre. In most case it's best to keep it as simple as possible and that means using VMDK unless you have a specific use case that dictates other wise. There are also a ton of post and documents regarding this same question if you want do to a few searches.

http://www.vmware.com/files/pdf/performance_char_vmfs_rdm.pdf

http://www.thevirtualblackhole.com/virtual-tech/to-use-rdms-or-not-to-use-rdms

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