Home > VMware, vSphere > Hot Add Memory on vSphere 4.1. Is it useful?

Hot Add Memory on vSphere 4.1. Is it useful?

Since vSphere 4.1, you can Hot Add memory inside a VM depending on the OS type.

I’m not going into the details on how to add the memory as that is described in numerous posts on the internet.


Let’s assume we have a Windows Server 2008 R2 with 5 GB of memory and a x64 application consuming 10 GB of RAM (we use TestLimit64 from sysinternals in this example, but typically this application could be Exchange, SQL, …).  I know, it sounds awful (as a matter of fact, it is awful 🙂 ).

If we look in Task Manager, we can see the server has severe memory shortage:


If we look into the details of our application using all this memory, we can see the following:


What does this mean?

– The process uses 10 GB of Virtual RAM.

– The process uses 4 GB of Physical RAM.

If we give the VM 16 GB of RAM (Hot Add), this can be seen in Task Manager:


We have more RAM available, but we aren’t using it!  The details of the process reveal this:


Still the same as before!  The VM has more physical RAM available, but the process isn’t using it!  If we restart the process, it does use the memory.


So basically, modern Operating System support Hot Adding memory, but very few application support this (to my knowledge SQL Server 2005 + ENT supports this).

So is it useful?  Yes, but not with the current amount of applications supporting this feature!

Categories: VMware, vSphere
  1. February 18, 2011 at 16:23

    I had no idea of that, but very good to know! Thanks!

  2. September 25, 2011 at 08:12

    This was an interesting article and it would be useful to learn more about the behavior of common guest applications.

    From my view it should work with most applications since they are using “virtual memory”, (not pagefile virtual memory, but memory virtualized by the operating system and that they typically do not “see” the physical memory). Since the application normally just see a large virtual memory area (2 GB for 32bit apps and around 8 TB for 64bit apps) they “should” be able to get more memory from the operating system if there is demand for it.

    (The “Hot add memory” feature was actually available in vSphere 4.0 already.)

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