4 Replies Latest reply on May 1, 2011 4:19 PM by neil_murphy

    Unexpected VM swapping

    neil_murphy Enthusiast



      I have a customer who is running some large VMs (8 vCPU, 32GB RAM) on a pair of IBM x3755 NUMA systems. Each NUMA node has 128GB of RAM and 8 cores for a total of 48 cores and 512GB RAM per server. Overall memory and CPU utilization levels are low and NUMA locality has been checked using resxtop and is good (99-100%).

      On just one ocassion there was a very brief spike in swapping activity for one VM (swap in rate:382KBps, swap out rate: 577KBps). The customer wants to know what may have caused this as there was very little activity on the system at this time. The VM is running Windows Server 2003 Enterprise, 64bit and SQL 2005.


      Has anyone any suggestions?


      Many thanks,



        • 1. Re: Unexpected VM swapping
          FranckRookie Virtuoso
          User Moderators

          Hi Neil,


          Is your system overcommitted? Have you grant your VMs more memory than you really have physically? I have seen some ESXs starting to swap even if the VMs were not using their memory but just because we gave them more than 140% of the physical capacity of the hosts.


          Good luck.





          • 2. Re: Unexpected VM swapping
            VMmatty Master

            Do you see any other signs that the host is constrained on memory?  Do you see any other ballooning or memory compression numbers?


            If not then maybe it was just a situation where VMs were rebooted at the same time and the host temporarily ran out of memory.  And I realize this is a stupid question but the swap values you're referencing there are on the ESX host and not inside th guest, correct?




            • 3. Re: Unexpected VM swapping
              neil_murphy Enthusiast

              Hi Matt,


              Correct - this was vmkernel swapping, not internal OS swapping.


              I know that there was a reboot of the system and all VMs at about 5PM that day but the brief spike in swap activity appeared about 1 hour previous to that.


              There was some balloon activity at the time but that was due to the customer setting a limit on memory of 16GB when the configured memory was 32GB. SQL was running and this scenario causes ballooning to occurr. But the system as a whole was not under memory constraint. In fact, I would say that it was only up to 50% usage at the time.



              • 4. Re: Unexpected VM swapping
                neil_murphy Enthusiast

                Hi Franck,


                No, usage was at max about 50% of the physical memory.