VMware vSphere

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  • 1.  Which metric to use for monitoring VM memory usage?

    Posted Sep 19, 2012 10:29 PM

    Hello all,

    For the moment, I'm getting headaches concerning the memory monitoring of the VMs on HA/DRS cluster.

    Regarding, VMware metrics, everything is going fine and I'm sure of that because on each host member of this cluster I have around 52% physical RAM free. It's designed this way for 2 reasons, we don't want memory over commitment and we want to limit performances impact in case of host failure.

    My main concern here is the the famous metric: Active Guest Memory.

    I don't understand why I have a so big difference between the Task Manager in Windows 2008 R2 VM ( Physical Memory Usage) and the "Active Guest Memory" metric in vSphere Client.

    To give you a better view on the topic that give me headaches, I will attach to this post 2 screenshot for the same VM. One of the taskmanager inside guest and the second is the metric in vSphere client.

    The concerned machine is a Windows 2008 R2 VM configured with 10GB of RAM and hosting several SQL DB.

    I've seen on google that I'm not alone in this situation.

    Can someone give me a clearer picture on this?

    Many thanks for your contribution,

    Michaël



  • 2.  RE: Which metric to use for monitoring VM memory usage?

    Posted Sep 19, 2012 11:21 PM

    You simply can't compare ESX(i) measured active memory with what you see in the guests task manager.

    It's normal for applications like SQL Server to hord as much memory as it gets in the guest OS, unless you limit it within SQL. But just because the memory is allocated in the guest OS does not mean anything about how much is really being actively used and thus doing anything useful. You could allocate 64GB to the VM, run no database at all (apart from the system databases), and you would still see that SQL hogs 62GB of memory in task manager, while active memory in vSphere is maybe ~1GB.

    ESX(i) active memory is calculated by the amount of pages that have recently been touched by the guest.

    For example, if you reboot a Windows machine, you will briefly see about 100% active memory because Windows zeros all memory pages at boot time.

    In your case, the system is just not actively using as much as it is being allocated.

    See this post for some more explanations on memory metrics:

    http://www.yellow-bricks.com/2010/12/20/vcenter-and-memory-metrics/



  • 3.  RE: Which metric to use for monitoring VM memory usage?

    Posted Sep 20, 2012 07:38 PM

    Hi,

    Thanks your input, it seems more clearer. It seems that I didn't catch correctly the meaning of "Active"