When it comes to disk configuration options for virtualized servers running on VMware, you have two options: VMFS virtual file-based disk (VMDK) or a raw device mapping (RDM). Which one do you need?
VMware has published reports that suggest that, with vSphere 5.1, there is little difference in performance
between the two options. If performance is not a factor, then what other differences exist between the two such that you may choose one versus the other?
First, let’s look at the nature of each option:
- VMDK—This is a file that appears as a hard drive to the guest operating system. Essentially, it is a virtual hard drive.
- RDM—This is also known as a pass-thru disk, and is a mapping file that acts as a proxy for a physical device such as a LUN.
Both options offer many of the same features such as file locking, permissions, persistent naming, and the ability to leverage vMotion.
The default recommendation for VMware 5.1 is to use VMDK for most virtual guests, including database servers. In fact, you should only use RDM for one of the following reasons:
- If you need files larger than 2TB in size, as VMDK files are limited to 2TB in size.
- Use of SAN snapshots based upon hardware Volume Shadow Copy Service (VSS). [Some storage providers are starting to allow for the use of VMDKs here, so check with your vendor.]
- Requirement to use clustered data and quorum disks (applies for both virtual-to-virtual clusters as well as physical-to-virtual).
While earlier versions of VMware offered differences in performance between VMDK and RDM, today the decision between the two options should be one of business requirements and architecture.