Section 4.6. Raw Device Mapping

4.6. Raw Device Mapping

New in ESX Server 2.5, raw device mapping allows you to access a raw physical device such as a SAN LUN. Through the use of a symbolic link in the VMFS partition, a mapping is created to the raw device. A common use for mapping raw devices is to support Microsoft Cluster Service, which we will discuss in the next section. But first let's drill down a little bit more into raw device mapping.

Prior releases of ESX Server supported accessing raw disk, but if changes were made, say, adding an additional HBA or SCSI card and the order in which ESX Server read these cards changed, disk resources were made unavailable. Through tools like vmkpcidivyi you could generally recover, but it was a pain, and you suffered obvious downtime.

In ESX Server 2.5, raw device mapping helps with this. So what is a raw device mapping? As mentioned already in this chapter, it is basically a symbolic link (or mapping file) that is in the VMFS partition and has a .vmdk extension. The mapping file includes metadata that is used to "manage and redirect disk accesses to the physical device," according to VMware documentation.

If you are clustering using Microsoft Cluster Service or taking SAN snapshots in virtual machines, VMware recommends using raw device mapping for those VMs.

Two compatibility modes exist for utilizing device mapping: virtual and physical.

Virtual mode makes raw device mapping act as a traditional .vmdk. This mode allows options such as redo logs and advanced file locking, and allows for greater portability over SAN hardware.

Physical mode allows greater flexibility with SAN software, allowing for SAN snaps in the virtual machine. This mode also provides the option of virtual to physical clustering.

See the following white paper for more details on raw device mapping at www.vmware.com/pdf/esx25_rawdevicemapping.pdf.