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Could anyone suggest me a hypervisor that permits adding RAM to a running virtual machine without restarting the guest OS. Is this even possible? Can this be done on xen? I have never found any documentation for such a thing.

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5 Answers 5

up vote 4 down vote accepted

Xen,KVM,Hyper-V,Vmware all support increasing RAM with out restarting.

Its done through balloon drivers.

check for its support on Guest

cat /boot/config-`uname -r` |grep -i balloon

In my VM it shows

CONFIG_VMWARE_BALLOON=m

CONFIG_XEN_BALLOON=y

CONFIG_HYPERV_BALLOON=m

CONFIG_VIRTIO_BALLOON=m 

But for ballooning RAM up,you need to set max RAM on guests,with out this value you can only decrease RAM but can not increase RAM.(at least in Xen and KVM it is so not sure how to do it in Vmware and Hyper-V)

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You can hot-add memory with vSphere. However, there is another question you should be asking: "What OS supports the hot addition of RAM?" The guest OS will have to have the ability to see RAM that has been added without a reboot. For example, Windows Server 2003 Enterprise (not standard) supports this, and all Server 2008 versions do as well.

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I have hot added ram to Centos 5.7 and above.. (not sure if it works below that or not, that is my oldest VM I have tried it on) –  Brian Apr 29 '12 at 6:26
    
Microsoft Hyper-V allows hot RAM add as well as dynamic RAM. But @wesleydavid is correct, the guest OD needs the feature as well. –  uSlackr Apr 29 '12 at 15:00

As nice as the hot-add option is, I wouldn't base a hypervisor choice off of that feature. Are you unable to acquire the downtime needed to make a quick configuration change to size your VM's RAM appropriately?

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This is more for a desktop OS being virtualized. If I am running multiple OS and shut down one, I would like to dedicate the RAM to other running OS. The OS is most likely going to be running some code (simulation) that may run for 10s of hours to days. –  Lord Loh. Apr 30 '12 at 6:37
    
That does not require any hot adding though, just rebalancing up to the maximum available at start. –  TomTom Jun 16 at 8:12

See my question and the answers/comments. Yes. XEN can do this. But it depends - both ends have to support it.

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When the VM was created, you must have enabled the "Virtual CPU hot add" and "Memory hot add" features.

The VM guest OS must support hot-add for CPU and RAM.

The VM guest OS must have VM Tools installed.

If your VM does not have the 'Virtual CPU hot add" and "Memory hot add" features enabled, you must power off the VM to enable them.

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Where is that quote from? Which hypervisor are you recommending? –  Felix Frank Jun 16 at 8:48

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