Please clarify downside to RDM, and how do I change vmfs lun to RDM?

[REALM]

I read through many threads and still don't fully understand the downside to RDM.

#1 So technically the VM just has a pointer to the SAN lun with RDM, correct? And I could present that RDM disk easily to any server and see the data, and there may be a slight performance advantage with RDM.

If that's the case, what is the advantage of VMFS over RDM with a SAN?

#2 Can a san LUN be broken up into multiple RDM disks going to different VM's?

#3 If I have an unused VMFS san lun of 600GB, and want to make that an RDM lun, how do I do that? Do I have to fdisk and unformat the thing first? Then remove from storage and re-add?

Using ESX 3.5 Update 2 and vCenter 2.5 Update 2.

Thank you.

[vmroyale · Accepted solution]

#1 So technically the VM just has a pointer to the SAN lun with RDM, correct? And I could present that RDM disk easily to any server and see the data, and there may be a slight performance advantage with RDM.

Correct.

If that's the case, what is the advantage of VMFS over RDM with a SAN?

Advantages of VMFS are that you can put lots of virtual disks on the one volume. Its a one-to-many relationship where the RDM is a one-to-one relationship of one LUN per VM. There is currently a maximum limit of 256 LUNs per ESX host, so if you had a lot of VMs you could get into trouble this way. 3 RDMs per 100 hosts would be a problem. Another consideration is that the more RDMs you add, the more you complicate the management of your storage environment. Its much easier to manage 3 or 4 LUNs than 30-40 of them. Typically RDMs are great when you need MSCS, want to leverage SAN backup options, or if you have LUNs that may need to be ported to a physical server at some point.

#2 Can a san LUN be broken up into multiple RDM disks going to different VM's?

I think you would want to break the LUNs up as well. Its much easier to manage those one-to-one relationships.

#3 If I have an unused VMFS san lun of 600GB, and want to make that an RDM lun, how do I do that? Do I have to fdisk and unformat the thing first? Then remove from storage and re-add?

Not exactly sure, but I would think you could remove the datastore from the ESX host, and then add it as a RDM to a virtual machine. fdisk should be able to fix it! But back to point 2 - it might make more management sense to carve that 600GB up and distribute it as needed.

Good Luck!

[mike_laspina]

Hi

The up side of vmfs based storage is the hardware autonomy factor and the ease of relocation.

Here is an example of what's possible using vmfs based storage.