Raw Device Mapping question

[Kevan]

Hi

I have been tasked with setting up a number of Windows 2008 R2 servers each running SQL 2008.

I was planning to add 2 Virtual Disks to each server. A C:\ drive of about 30Gb and a D:\ Drive of about 250Gb which will hold the database files.

My question is whether I should use RDM's for for the database drives or just create large VMDK files?

I will have the same question with regards to an Exchange 2010 server should I create a RDM for the database or keep everything as VMDK's.

All our datastores are on an EMC Clarion CX 300 fibre SAN.

We do have VMotion enabled on all our Virtual machines.

I have not created VDM's before so any help or hints appreciated.

Regards

Kevan

[mittim12]

I have read numerous post and whitepapers on this subject and the most compelling reason I have found to use RDM is if you will be leveraging your SAN tools. The performance between the two is negligible as seen in this older paper, http://www.vmware.com/files/pdf/vmfs_rdm_perf.pdf. In 95% of our SQL servers we are using VMFS and haven't experienced any issues at all.

If you found this or any other post helpful please consider the use of the Helpful/Correct buttons to award points

Twitter: http://twitter.com/mittim12

[vmroyale]

Hello.

I was planning to add 2 Virtual Disks to each server. A C:\ drive of about 30Gb and a D:\ Drive of about 250Gb which will hold the database files.

It might be closer to best practices to create 3 drives. One for the OS, one for the logs and one for the DBs with each on the appropriate RAID levels.

My question is whether I should use RDM's for for the database drives or just create large VMDK files?

If performance is the reason, then you can go either route. If you intend to leverage features on the SAN for snapshots, backups, etc, then the RDM might make more sense. RDMs are also nice, if you may ever have to mount these disks back on physical servers.

We do have VMotion enabled on all our Virtual machines.

vMotion will work fine with either VMDK or RDMs.

Good Luck!

[JonT]

To go along with all of this if you plan to use VMDK files on a VMFS datastore, make sure that you plan to not share that volume with too many other VMDK's. That actually can ding your performance some. I did some basic performance benchmarks on VMDK disks and RDM disks for our SQL servers and noticed a difference in IOPs of about 10%, and about 15% difference in seek time (in ms) as well. Granted the VMDK in my tests were on a pretty busy shared datastore so thats why I would recommend using a more dedicated datastore approach.

To kind of head this off in our environment, I have a soft-limit on size of VMDK disks at 200GB. Anything larger than that, we enforce an RDM rule on customers/applications. This also buys us the SAN tools/features as mentioned below for things like using SRDF for replication and snapshots, to name a few.