Time after time this same question seems to just pop up over and over again.  Should I use Raw Device Mappings or RDM’s and if so, when should I use them?  From a strictly performance point of view vmdk files and RDM are just about equal.  The speed of the disk is directly related to the number of spindles that have been presented in the array.  From a management point of view the size of the LUN and or vmdk file plays the biggest part in my decision making process.  As a general rule, 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.  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.  To take this scenario a step further, if you have your backup tape device on the fiber then you could back up that data much quicker than going through the vmdk itself.

To give another example for reasons that I have chosen to use RDM’s over vmdk in particular, was a Microsoft Exchange environment that I deployed.  We worked out the number of spindles that we would use that was a direct match to the physical environment that we were building onto.  When I was presented the LUNs, they were the exact size that was requested and again a duplicate of the physical environment.  If I was going to use vmdk’s I would have lost valuable disk space for the headroom that is needed on a LUN, best practice is to leave around 20% of free space on the LUN and also leave room for delta files.  If all of the virtual machine files were stored together then there would also need to be room for the virtual machine’s swap file in which the size is equivalent to the amount of memory assigned to the virtual machine.  VMware’s ESX server will also add 512 MB’s in configuration files that are created when ESX creates the VMFS file system. 

To summarize always remember to K.I.S.S. or Keep It Simple Silly.  Use vmdk files unless you find a specific reason that you would need to do something different.  Reasons do pop up from time to time but they are the exception to the rule generally.

RDM, virtualization, VMDK, vmware

This entry was posted on Tuesday, March 24th, 2009 at 12:13 pm and is filed under Virtual Tech. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.