2 Replies Latest reply on Feb 9, 2011 11:23 AM by patrickds

    Disk I/O statistics after consolidation

    patrickds Expert

      Whenever i run a consolidation scenario, the disk and network I/O statistics show lower Transfers/sec and Bytes/sec on the virtualized machines, compared to the original.


      What are these numbers based on?

      Would a physical machine, when virtualized, suddenly become less disk- or network-intensive?


      Even when VMs are included in the assessment, their numbers are halved, as if a redeploying a VM to a new host would make it generate only half the I/Os as before.



                    Disk I/O Trans/sec     Disk I/O(MB/sec)     Network I/O(MB/sec)

      Before:      3,645.41                             105.25       3.53

      After:        2,651.57                              73.241       2.816


      Where does this difference come from?

        • 1. Re: Disk I/O statistics after consolidation
          Suzan Enthusiast

          This may be due to the fact that they are in VMs.



          If the tool is measuring actual I/O on the card, some of this will be reduced due to the fact that you are now taking advantage of a logical network built into your host to talk to other VMs on the same host.


          Disk I/O:

          Moving from local disk on a physical server to a VM makes your disk I/O occur on the SAN, between .vmdk files.  This reduces the disk I/O partially because of the improved capabilities of the disks in your SAN, and partially because you're now transferring (potentially) from file to file rather than from disk to disk.

          • 2. Re: Disk I/O statistics after consolidation
            patrickds Expert

            Typically, most of the network IO would be between VMs and systems outside of the virtual infrastructure.

            Lowering network traffic by almost 50% to account for inter-VM traffic doesn't make much sense.

            Only traffic between VMs on the same host and vswitch would not generate any external I/O.


            Same for disk IO: most of the data written to or read from a VM will originate from or get sent to an external source, not the VM itself.

            File copies between disks in a single VM (or on a physical server actually) would not occur very frequently.