6 Replies Latest reply on Aug 25, 2006 5:19 AM by rvanthiel

    help me understand host/guest memory usage

    rvanthiel Enthusiast

      VI3 VC2


      Hello -


      I'm trying to get a grip on on the "host memory usage" and "guest memory usage" stats as presented in VC.  Perhaps someone can explain or point me to the correct documentation please.


      How I currently understand it:


      The "host memory usage" number refers to the amount of physical memory a VM is actually using (including its overhead memory usage), factoring in shared pages and such. Easy enough.


      "Guest memory usage is the amount of actively used memory by the VM."  This is where it's losing me.  If VC is reporting say 17MB of guest memory usage, how does it come up with this?  I would think this would be closer to what the guest operating system would then report, but obviously it's not.  The guest OS reports about 100MB memory currently used.  The "host memory usage" reads 150MB with the overhead, etc.  I'm trying to understand what "guest memory usage" then represents.


      I'm not really troubleshooting a specific problem or anything, because then I would get into esxtop and such.  For now, I'm just trying to understand what VC is showing me.



        • 1. Re: help me understand host/guest memory usage
          phenrichs Hot Shot

          I have found that when virtual memory is assigned to a vm the system assigns half of it to physical memory and half to the swap file memory.  You also have the option of reserving physical memory for each vm.  This means that there must be an amount of physical memory equal to the reservation free in order to power on the vm.  This does not mean that the vm will use all of the memory however, if you assign 1GB but the vm only uses 512 the system can reallocate the other 512 somewhere else.


          Hope this helps.

          • 2. Re: help me understand host/guest memory usage
            rvanthiel Enthusiast

            The "official" def from VMware:


            "Guest memory usage is based on the amount of memory that is actively being used by a process.  If you would like to test this, simply run a batch loop script inside the Virtual Machine and you will notice that the Guest memory usage will increase."


            I don't see the point; the "host memory usage" seems more beneficial to me.  But that's just me

            • 3. Re: help me understand host/guest memory usage
              shaneyoder Hot Shot

              I like the Guest Memory Usage better. Take an example of a SQL/Exchange server (which will chew up all the RAM that it can whether it needs it or not) with 2GB assigned to the VM. the Host Memory usage will show almost all 2GB in use, while the Guest Memory Usage will show you how much of that 2GB is truly needed and actively used and not just "held" by the application.


              I have a 2003 Exchange server with 4 mailboxes that shows 1.9GB Host memory usage but the Guest usage only shows 5% (which is truly how much memory is needed). I just haven't migrated the other 200 mailboxes yet, so I know the guest memory usage will go up considerably.


              Now I'm not saying to drop your RAM for each VM so the guest memory usage is 80%, but there is no need in wasting RAM on a VM just because certain applications will take whatever you give it and then Task Manager shows you only have a few hundred MBs left. Just watch your performance charts and you'll quickly see what VMs are truly using the resources and which ones aren't.

              1 person found this helpful
              • 4. Re: help me understand host/guest memory usage
                rvanthiel Enthusiast

                Ahhhh, ok....now we're starting to make sense.  I was trying to understand the application of the "guest memory usage" stat, and that definitely applies.  I guess I have yet to see that scenario myself, but thank you.

                • 5. Re: help me understand host/guest memory usage
                  crashradtke Novice



                  What is the Host Memory Usage claiming when it is in the ESX server pane itself?


                  I am seeing errors and the previous explanation of host/guest has helped me tune the minimum requirements for each machine so that they have adequate resorces (thanks!


                  Message was edited by:


                  • 6. Re: help me understand host/guest memory usage
                    rvanthiel Enthusiast

                    crashradtke -


                    Then you're looking at the host server total memory utilization.  That would be the physical RAM in use, which would encompass all virtual machines running, the virtualization overhead, the service console, etc.  If you're consistently spiking that and generating warnings, you either need to decrease the memory allocation for some virtual machines or add more physical memory to your server.