Adding Raw Device Mappings for Maximum Performance

We know that adding an RDM to your SQL and Exchange hosts can have an enormous impact on IO performance. In addition, under Windows Server guests prior to Server 2008, the partition must be properly aligned when accessing a RAID volume in order to achieve maximum performance. This is due to the fact that with a physical disk that maintains 64 sectors per track, Windows always creates the partition starting at the sixty-forth sector, therefore misaligning it with the underlying physical disk. In some cases, aligning a partition can boost IO performance by up to 50 percent! Here is a step by step guide to creating, attaching and aligning an RDM partition in ESX 3.5 on Server 2003.

Attaching a Raw Device Mapping

1. Log on to your SAN and create your LUN. Make this LUN available to all ESX hosts in your cluster. The steps needed to do this will vary by SAN and Fibre switch, so consult your vendor’s documentation for more info.
2. Log in to Virtual Infrastructure Client and connect to your Virtual Center instance.
3. Click on an ESX host and choose the Configuration tab.
4. Click on Storage adapters and rescan for new storage devices. Your new LUNs should show up. You do NOT want to create a VMFS here.
5. Repeat this procedure for each ESX host in your cluster.
6. Right click on the guest OS that you will be attaching the RDM to and click on Edit Settings.
7. Click Add in the hardware tab, choose Hard Disk and then Raw Device mapping.

Adding your new disk to the guest OS

1. Log on to your guest OS and launch Device Manager.
2. Scan for hardware changes.
3. Open Disk Management and initialize the new disk. Do not create a partition at this time.

Adding a properly aligned partition to the RDM

1. Log on to the server.
2. Type diskpart at the command line to launch the diskpart utility.
3. Type list disk to see a list of disks present. For my example, I will be creating a partition on Disk 2 and it will be the only partition on this disk.
4. Type select . In my example, you would type SELECT Disk 2.
5. Type create partition primary align=64 to create a primary partition that takes up the entire disk. You can use the size keyword if you are creating more than one partition on the disk.
6. After you have finished here, you will need to go in to Disk Management, format the partition and assign it a drive letter as you would normally.