RDM or VMDK - advice needed

[big_vern]

Hi ,

we have a HP EVA 8000 SAN, I am setting up a File / Print server (1.2 TB is size). I am thinking of using a RDM for the data disk

The reasoning is:

We can present to a physical box should the need arise (testing / performance issues)

If the management change their mind and want service level recovery we can fall back to Microsoft Clustering

SAN snapshots - we can then present this snapshot to a Windows box for backups etc (we have found this to be quicker than using vcb mounter on the same server)

Is this correct ?

My next is - physical or virtual compatability mode ?

Is physical only required if you arte using Software inside the VM that required direct scsi access (ie, it wont affect storage array snapshots ?) - is this the only reason to use physical ?

cheers guys, feel free to shoot me down - I've only had time for a quick read but need to make a quick decision - as usual !

[Texiwill]

Hello,

we have a HP EVA 8000 SAN, I am setting up a File / Print server (1.2 TB is size). I am thinking of using a RDM for the data disk

We can present to a physical box should the need arise (testing / performance issues)

Absolutely a good choice for something that large. While there is no real performance difference between an RDM and VMDK, something that size you want on its on RAID5 LUN that you can use anywhere.

If the management change their mind and want service level recovery we can fall back to Microsoft Clustering

Actually you can use MSCS within ESX if the data is an RDM. THat is one of the MSCS requirements for ESX, all but the boot drives of the cluster nodes should be RDMs.

SAN snapshots - we can then present this snapshot to a Windows box for backups etc (we have found this to be quicker than using vcb mounter on the same server)

This is also a very good reason, yes, it will be quicker than VCB as the snapshot is happening within the data storage level. However, a LUN snapshot produces a crash consistent copy of the LUN. You may still want to quiesce the LUN before snapping it off the mirror....

My next is - physical or virtual compatability mode ?

Physical should be used if the LUN will be part of a MSCS that includes external hardware. Virtual will allow you to vMotion the VM. Within ESX virtual gives an RDM all the properties of a VMDK yet it is not a VMDK. There is still no major difference between and RDM and VMDK w/regards to performance.

Best regards,

Edward L. Haletky, author of the forthcoming 'VMWare ESX Server in the Enterprise: Planning and Securing Virtualization Servers', publishing January 2008, (c) 2008 Pearson Education. Available on Rough Cuts at http://safari.informit.com/9780132302074

[virtualdud3]

Here is a link to a whitepaper you might find useful:

http://www.vmware.com/pdf/vmfs-best-practices-wp.pdf

[big_vern]

Thanks Texiwill,

Are you saying that Vmotion will not work with Physical compatability mode ? , according to the doc link posted above it does (page 12)

[Texiwill · Accepted solution]

Hello,

All RDMs can be used for vMotion, or they can now, at one point only virtual did (some form of ESX v2 however). My mistype. I tend to keep things virtual unless I am using them with physical nodes. Just so that if this changes it does not break. I can also snapshot my virtual RDMs, not sure you should but you can.

Best regards,

Edward L. Haletky, author of the forthcoming 'VMWare ESX Server in the Enterprise: Planning and Securing Virtualization Servers', publishing January 2008, (c) 2008 Pearson Education. Available on Rough Cuts at http://safari.informit.com/9780132302074

[big_vern]

Ok - thanks for the advice - I'm in a much better posittion now to validate my decisions.

:smileyblush: