VMDK versus RDM: Which One Do I Need for SQL Server?

March 26, 2014

VMDK versus RDM: Which One Do I Need for SQL Server?

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.


Thomas LaRock is a Head Geek at SolarWinds and a Microsoft® Certified Master, Microsoft Data Platform MVP, VMware® vExpert, and former Microsoft Certified Trainer. He has over 20 years’ experience in the IT industry as a programmer, developer, analyst, and database administrator. LaRock has spent much of his career focused on data and database administration, which led to his being chosen as a Technical Evangelist for Confio Software in 2010, where his research and experience helped to create the initial versions of the software now known as SolarWinds® Database Performance Analyzer. LaRock has served on the board of directors for the Professional Association for SQL Server® (PASS), and is an avid blogger, author, and technical reviewer for numerous books about SQL Server management. He now focuses on working with customers to help resolve problems and answer questions regarding database performance tuning and virtualization for SQL Server, Oracle®, MySQL®, SAP®, and DB2®. He’s made it his mission to give IT and data professionals longer weekends.