4 Replies Latest reply on Sep 29, 2009 9:13 PM by dquintana

    vmdk vs RDM for large disk

    jf2000 Novice


      environment is ESX3.0.2 and VC 2.0.2.  EMC symetrix storage.



      I need to provision out a VM w/ a 600GB secondary disk.  Anyone have any recommendations (pros/cons) on which would be better, a vmdk file or RDM?



      PROs for vmdk are vmotioning and expanding, but can an RDM, if presented to the rest of the host, be vmotioned around?  And is there any benchmarks that say one is better then the other for disk read/writes?






        • 1. Re: vmdk vs RDM for large disk
          vmroyale Guru


          RDMs tend to find uses in Microsoft Cluster Service (MSCS) or if you run SAN snapshot or other layered applications in the virtual machine. RDMs better enable systems to use the hardware features inherent to SAN arrays.  If you don't have of these requirements then VMFS will be fine. 



          You can use VMotion with RDMs. 



          If you use RDMs, also think a bit about the compatibility mode (Virtual vs Physical).  If you use Physical, then you won't be able to use snapshots - this may present problems for your backup strategy.



          As for the performance, check out "Performance Characteristics of VMFS and RDM" at:




          With all that said, I tend to stick with VMFS volumes most of the time.



          • 2. Re: vmdk vs RDM for large disk
            Phudodragon Novice


            I've got four different VM's that have 750GB of space on data drives. Two of them use VMFS and the other two are RDM.  I tend to use VMFS for our development and test environments and RDM for production level environments.  The reason for this distinction is that with our NetApp SAN we can take advantage of the snapshot and restore capabilities provided by it to quickly restore the data drives in our produciton environment. 



            One of the things I do like about the VMFS volume is that it is true space when you allocate it. On a SAN there is always overhead when allocating space, sometimes by as much as 50% depending on the settinngs you have.  On VMFS, if you aren't using VMware snapshots, then your 600 GB allocated is just 600 GB.  For my VMFS volumes on a SAN I remove all the overhead. 



            Also, at least with the NetApp, for space savings we take advantage of flexclones (writeable snapshots) and mount these to any DEV and TEST environment we can. With just the changes made consuming space, massive storage savings can be gained.  Also, this allows us to use scripts to create immediate copies of our production envrionments which give the developers access to real data to test against.  We even scedule nightly flexclones to a "BETA" environment in order to always have fresh data available for our developers.  I'm not certain if EMC has anything like this, but if they do, definitely take advantage of it.








            • 3. Re: vmdk vs RDM for large disk
              ewannema Hot Shot

              There is a presentation from Blue Lock on one of the VMworld open sessions where I believe they got almost native performance by using iSCSI in the guest to connect to the data hosted by their SAN.  It performed much better than vmdk or RDM.  Check the video out to make sure I remember correctly and test this scenario if iSCSI is an option.

              • 4. Re: vmdk vs RDM for large disk
                dquintana Master


                Excelent PDF.