Raw Device Mapping – RDM in vSphere 4 and 5 Resources and Experiences

RDM (Raw Device Mapping) has changed a little between VI and vSphere. They have continued to improve over time as well. Now there are a lot of resources out there for RDMs in general but I just wanted to do my own research and learn about the particulars. I also wanted to note some of the blogs I used and the difference between vSphere 4 and vSphere 5. I don’t think things have changed a whole lot for RDM’s other then the size of pRDM (Physical or Pass-thru) have been increased almost to 64TB.

Now RDM is more of a topic of “use case” than anything. Other things like “Performance” can be debated on all kinds of levels and usually there are always workarounds to avoiding using RDM’s. Now the real limitation to RDMs in my opinion is pretty much not being able to back it up with the vStorage API. That is my only real personal gripe. The size of an RDM can sometimes be more accommodating for large Databases and large file servers in some cases. Again this is “use case”. For the most part when I see RDMs implemented in our environment it was done based off of bad logic… meaning some people don’t understand the risk associated with what they are doing or wasn’t necessarily the best “use case” they intended it for. Mostly when I see that a pRDM is used a vRDM would’ve worked better.

A misconception: “RDM’s are more secure because you cannot snapshot”

I am not sure where I can start this discussion. This was what a colleague of mine told me around the first time I started learning vSphere 4 and I kind of believed him at first but he too was relatively new and I know being in a virtual environment there is a way around anything..almost.

This was a hot topic because we were virtualizing AD servers and they were arguing with the AD team about how they should implement AD on vSphere. The security team didn’t want anything to do with it and they didn’t want to go along with it at all. So the engineer(s) decided to throw out the RDM as a way to keep people from being able to snapshot AD so they could keep AD from blowing up (Misconception as well) and securing the data because you couldn’t just copy it off. What they were thinking was that if they cannot actually see the VMDK file then no one can steal or retrieve the data. What a misconception that is… there are many ways to convert any RDM into a VMDK file as long as it’s not over a particular size that is – 2TB – 512mb.

You can use a variety of tools from VMKFSTools, Cold Migrations, sVmotion (vRDM) and cloning. Of course there are limitations like snapshots and such which can prevent this from happening. What you need to note is that if you are using a RDM for a particular case make sure you DON’T accidently convert it to a VMDK file.

The following reference is from a KB located here:

Cold Migration

With file relocation:

  • Any non-RDM virtual disks are physically moved to the destination.
  • The virtual machine configuration files are physically moved to the destination.
  • Raw LUNs themselves cannot be moved, as they are raw disks presented from the SAN. However the pointer files (RDMs) can be relocated if required.
  • When performing a cold migration of a virtual machine with RDMs attached to it, the contents of the raw LUN mapped by the RDM are copied into a new .vmdk file at the destination, effectively converting or cloning a raw LUN into a virtual disk. This also applies when the virtual machine is not moving between ESX hosts. In this process, your original raw LUN is left intact. However, the virtual machine no longer reads or writes to it. Instead, the newly-created virtual disk is used.
  • If you wish to cold migrate a virtual machine without cloning or converting its RDMs, remove them from the configuration of the virtual machine before migrating. You can delete the RDM from the disk when removing it (the raw LUN contents are not changed). Re-add them to the configuration when completed.

Again other references can be noted in the KB. Also if you wanted to convert your RDM from Physical to virtual mode just follow this KB here. Also Scott Lowe did a great job writing about this similar subject in regards to svMotion here.

In short, I just wanted to know a particular reference and the misconception of RDM’s being secure because you can easily convert an RDM to a VMDK and VMDK to RDM.

I would like to thank VMware and all the VMware Bloggers if you know of any other good resources let me know and I’ll update it.

Resources vSphere 4.1: (pg.28) (Lots of references to RDM) (Lots of Good stuff) (RDM performance Blog) (Great resource for learning RDMs and Conversions) (RDM Performance Article) (Scott Lowe’s Blog on sVmotion RDM)

Resources vSphere 5: (pg. 39) (pg. 140)

***Disclaimer: The thoughts and views expressed on  and Chad King in no way reflect the views or thoughts of his employer or any other views of a company. These are his personal opinions which are formed on his own. Also, products improve over time and some things maybe out of date. Please feel free to contact us and request an update and we will be happy to assist. Thanks!~


About Chad King

I am an IT professional working in the industry for over 10 years. Starting in Microsoft Administration and Solutions I was also a free lance consultant for small businesses. Since I first saw virtualization I have always been fascinated by the concept. I currently specialize in VMware technology. I consult daily on many different types of VMware Solutions. I have experience in all domains related to virtualization and cloud solutions. From help desk, administration, engineering, implementation, and design allowing me to provide creative solutions with the understanding and gaps at all levels.

Posted on October 28, 2011, in vSphere 4, vSphere 5 and tagged , , , , , , , , , , . Bookmark the permalink. 2 Comments.

  1. I am truly delighted to read this blog posts which consists of tons of helpful information,
    thanks for providing these information.


  2. Way cool! Some very valid points! I appreciate you penning
    this write-up plus the rest of the website is very good.


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