VMware
A Raw Disk Mapping (RDM) can be used to present a LUN directly to a virtual machine from a SAN. Rather than creating a virtual disk (VMDK) on a LUN, which is generally shared with other VMs and virtual disks.
The reasons for doing this should purley be for functional and management reasons, NOT performance. There is a mis-understanding that RDMs offer greater performance compared to VMDK's on a VMFS datastore. I've seen lots of vSphere environments that have gone over kill on RDMs for SQL servers and the like for "performance reasons", its difficult to manage! If your looking for improved storage performance look into the VMware Paravirtual SCSI (PVSCSI) adaptor.
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All advice, installation/configuration how to guides, troubleshooting and other information on this website are provided as-is with no warranty or guarantee. Whilst the information provided is correct to the best of my knowledge, I am not reponsible for any issues that may arise using this information, and you do so at your own risk. As always before performing anything; check, double check, test and always ensure you have a backup.