Raw Device Mapping (RDM) Questions

[bgushue]

The scenario (required):

1 vm which will act as a file share. It will require one OS drive and a data drive of 200 GB. It is possible it may require expansion in the future.

3 vms which will act as XenApp servers. They will require 80 GB of space each (only one drive).

We're wondering if RDM would be a good option for either or all of these servers. We've only used VMFS to this point, but are interested in the advantages of RDM.

Questions:

1. I've been told RDM would improve I/O on systems that require high I/O (eg., SQL) so would I see any advantage on a straight file share? How about on XenApp servers?

2. RDMs have to be added as one full LUN, correct? In other words, when I create a LUN on the storage (Dell MD3000i) it will be presented to the ESX hosts and when I create it on the vm, I will only have the choice to add the whole LUN as a RDM? Ideally I would like to create one whole disk group on the MD3000i and carve it up. Would I have to create 4 LUNs (1 @ 200 GB and 3 @ 80 GB)?

3. How would I expand the data drive on the file share RDM, if I ever had to?

Thanks.

[Lightbulb · Accepted solution]

1. I've been told RDM would improve I/O on systems that require high I/O (eg., SQL) so would I see any advantage on a straight file share? How about on XenApp servers? Meh I would go with VMFS for the Fileserver no real advantage with an RDM here. I really can't say with regards Xenapp systems but unless the app provider says otherwise I would stick with VMFS

2. RDMs have to be added as one full LUN, correct? In other words, when I create a LUN on the storage (Dell MD3000i) it will be presented to the ESX hosts and when I create it on the vm, I will only have the choice to add the whole LUN as a RDM? Ideally I would like to create one whole disk group on the MD3000i and carve it up. Would I have to create 4 LUNs (1 @ 200 GB and 3 @ 80 GB)? Full LUN, yes

3. How would I expand the data drive on the file share RDM, if I ever had to? IIRC you would expand the LUN on storage, shutdown the guest and recreate the RDM mapping. Start up the VM and the guest should see the space.

[okeedokee]

3. How would I expand the data drive on the file share RDM, if I ever had to? IIRC you would expand the LUN on storage, shutdown the guest and recreate the RDM mapping (be sure to rescan your storage adapters, twice if you have multiple paths). Start up the VM and the guest should see the space.

[bgushue]

Thank you both for replying.

I have a little more information on the File Share server. It is going to hold roaming profiles, re-directed folders and terminal services roaming profiles. So it appears it will be a little more than just a straight file share. Does this change anything?

So it seems you should use vmfs unless you have good reason to go to RDM, correct? If so, why? Flexibility?

Thank you.

[Lightbulb]

Go with VMFS not an RDM. Generaly you go with RDMs for a couple of reasons MSCS or SAN snapshots or high performance filesystem (This is debatable). in your case this does not come in to play. Also VMFS is simpler fewer LUNS to manage.

[AndreTheGiant]

So it seems you should use vmfs unless you have good reason to go to RDM, correct?

Yes.

If so, why? Flexibility?

Simplicity: all the VM is in a folder.

RDM could be usefull to P2V (if the physical machine is already connected to the SAN) or Application Cluster.

But usally RDM is not necessary.

Andre

**if you found this or any other answer useful please consider allocating points for helpful or correct answers

[okeedokee]

So it appears it will be a little more than just a straight file share. Does this change anything?

No, I think you'll be good using VMFS. Using the volume for profile data will cause it to be more active then a share solely for documents, but I don't think this level of activity necessitates using RDM.

Regards

[bgushue]

Thank you all for the responses. We are following your advice and using vmfs.

[karemabdou]

how can i add the same LUN ( Raw Device Mapping (RDM) to 2 VMS.