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Use RDMs for Practical Reasons and Not Performance Reasons

April 13th, 2009 |

Author: Rich Brambley

“Should I use VMFS or RDMs (Raw Device Mappings) for my VMware ESX virtual machines (VMs)?” It’s a popular discussion not just for new virtual infrastructure (VI) planning but for existing environments as well. In my experience, Many VMware ESX virtual machines using RDMs were created because of a perceived boost in VM and application performance. Somehow, the wrong impression became widespread: the proprietary VMware VMFS file system created too much I/O overhead and slowed down the VM performance. Although the VMFS reputation has been publicly repaired again, there are still regular challenges explaining this virtual disk myth to VMware administrators.

There are definite pros and cons for using both VMFS and RDMs. This post suggests the 3 most common practical reasons (in my opinion) to use RDMs. That is, reasons that benefit VI administrators, leverage a VM’s native operating system, or take advantage of technologies and process designed for physical environments. I also provide some links to a few other blogs that have recently explored the same RDM vs VMFS topic as supporting examples.

First, if proof is still needed that performance should not be a factor in the VMFS versus RDM decision, check out VMware’s own PDF titled Performance Characteristics of VMFS and RDM.

So what are some practical reason for using RDMs?

Use RDMs when VMFS virtual disk would become too large to effectively manage.

For example, When a VM needs a partition that is greater than the VMFS 2 TB limit is a reason to use an RDM. Large file servers, if you choose to encapsulate them as a VM, are a prime example. Perhaps a data warehouse application would be another.

Stephen Beaver provides his guidelines on when RDMs make sense for large file management in a recent post on the Virtual Black Hole blog:

To Use RDM’s or Not To Use RDM’s

“I try to use RDM’s on anything larger than one terabyte. This is strictly based on management of the data and nothing else. For example, let’s say we have a file server with a one terabyte data drive. If I ever have to move that virtual machine to another cluster or had to move to another LUN, or insert your own reason here, then how long would it take to move that vmdk file? Way too long is the correct answer.”

Leading into the next reason, if you know that you may have to move a large VM between LUNs, then an RDM may make your task a lot easier when the time comes. (if moving between LUNs on the same SAN)

Use RDMs to leverage native SAN tools

SAN snapshots, direct backups, performance monitoring, and SAN management are all possible reasons to consider RDMs.

Again, In the previously mentioned Virtual Black Hole post, Stephen used a virtualized instance of an Exchange Server to illustrate RDM advantages when using SAN snapshots:

“If that terabyte of data was on a RDM then I could use the native SAN tools to snap the LUN and move the data in a much quicker rate.”

Sean Clark also mentions RDMs for taking advantage of SAN utilities in his post VMFS vs RDM? Does Miller Lite taste great or is it less filling?

“…except in instances where we would like to take advantage of application aware SAN utilities for backups and/or SAN snapshots.”

Use RDMs for virtualized MSCS Clusters

Actually, this is not a choice. Microsoft Clustering Services (MSCS) running on VMware VI require RDMs. Clustering VMs across ESX hosts is still commonly used when consolidating hardware to VI. The actual need for VM MSCS Clusters is debatable with VMotion and DRS/HA, but that’s another topic.

Jason Boche recently posted Setup for Microsoft cluster service which explains the needs for RDMs. Jason provides a great run through of all the requirements for MSCS on VMware as well.

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Use RDMs for Practical Reasons and Not Performance Reasons

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