pRDM vs vRDM virtual / physical interchangeability

[pinkerton]

Hi,

I'm currently implementing a Replication solution in our vSphere 5 environment and would like to be able to also replicate some VMs that currently use pRDMs. Since snapshots are a requirement, I know I need to change the corresponding pRDMs to vRDMs as described in http://kb.vmware.com/kb/1006599.

One reason for us to use pRDMs was the interchangeability with physical hosts. That is, if the VM goes down we could still present the important LUN to another (physical) host. And Scott Lowe's Book "Mastering VMware vSphere 5" states the following regarding the difference between pRDMs and vRDMs:

Another important use case of pRDMs is that they can be presented from a VM to a physical host interchangeably. This gives pRDMs a flexibility that isn't found with virtual mode RDMs or virtual disks.

However, I justed tested the interchangeability with a vRDM and found out that I can present a vRDM containing a NTFS parition to a physical host without any problems. Which in theory makes sense I guess, since both types of RDMs are - as far as I understand - just pointers to LUNs, with the difference that pRDMs submit all SCSI commands (with the exception of the REPORT LUNs command) to the device and vRDMs only submit the READ and WRITE commands to the device.

So is Scotts statement about vRDM interchangeability simply a small flaw in his book or am I missing something?

Thanks

Michael

[Mouhamad]

Hi Michael,

As far as I know, the main difference  between P and V RDMs is:

1st: Snapshots

2nd: The size which can go up to 64TB on vSphere 5 Physical RDM

I think the theory is not accurate.

[pinkerton]

Hi Mouhamad,

thanks, with theory you mean that interchangeability with vRDMs is not possible? I'm going to write Scott a mail, maybe it's really a small flaw in his (in general outstanding) book.

[Mouhamad]

Hi Mike,

You're right, it's a small flaw.  I've been working with RDMs for the last year and a half, and I was always able to present vRDMs to physical hosts.

Regards,

Mouhamad

[pinkerton]

Thanks Mouhamad,

then I'll go ahead and change the corresponding pRDMs to vRDMs.

Have a nice day

Michael