Specifying How Much RAM is Used by All Running Virtual Machines

You can set the amount of host RAM that GSX Server is allowed to reserve for all running virtual machines. To set this parameter, choose Host > Settings > Memory.

The reserved memory setting specifies a maximum amount of host RAM that GSX Server is allowed to use. But this memory is not allocated in advance. Even if multiple virtual machines are running at the same time, GSX Server may be using only a fraction of the RAM you specified here. Any unused RAM is available for use by other applications. If all the RAM you specify here is in use by one or more virtual machines, the host operating system cannot use this memory itself or allow other applications to use it.

Virtual machines require relatively large amounts of memory to operate with reasonable performance. An individual virtual machine can use at most the amount of memory specified in its configuration file plus some overhead. The amount of overhead memory required depends upon the size of the guest's virtual disks, its behavior and the amount of memory allocated to the virtual machine. Refer to the table below for the typical upper limit needed, based on the amount of memory allocated to the guest.


Amount of Memory Allocated to the Virtual Machine	Additional Amount of Overhead Needed
Up to 512MB	Up to 54MB
Up to 1000MB	Up to 62MB
Up to 2000MB	Up to 79MB
Up to 3600MB	Up to 105MB

The amount of RAM actually used for a particular virtual machine varies as a virtual machine runs. If multiple virtual machines run simultaneously, they work together to manage the memory.

The recommended amount of RAM to specify for all running virtual machines is calculated on the basis of the host computer's physical memory and is displayed in the memory settings slider control — Host > Settings > Memory. If you want GSX Server to use more or less physical memory, use this slider to change the amount.

If you set this value too high, the host may thrash when other applications are run on the host. If you set this value too low, virtual machines may perform poorly and you cannot run as many virtual machines at once.

By default, GSX Server limits the number of virtual machines that can run at once based on the amount of memory specified in the Host Settings dialog box. This limit prevents virtual machines from causing each other to perform poorly. If you try to power on a virtual machine and there is not enough memory available, a warning appears and the virtual machine fails to power on.

To increase the number or memory size of virtual machines that can run, adjust the amount of virtual machine memory that the host operating system may swap to disk. To change this setting, choose Host > Settings > Memory and choose an option under Additional memory.

Fit all virtual machine memory into reserved host RAM — Strictly apply the reserved memory limit set in the top of the dialog box. This setting imposes the tightest restrictions on the number and memory size of virtual machines that may run at a given time. Because the virtual machines are running entirely in RAM, they have the best possible performance.

Allow some virtual machine memory to be swapped — Allow the host operating system to swap a moderate amount of virtual machine memory to disk if necessary. This setting allows you to increase the number or memory size of virtual machines that can run on the host computer at a given time. It may also result in reduced performance if virtual machine memory must be shifted between RAM and disk.

Allow most virtual machine memory to be swapped — Allow the host operating system to swap as much virtual machine memory to disk as it wants. This setting allows you to run even more virtual machines with even more memory than the intermediate setting does. In this case, too, performance may be lower if virtual machine memory must be shifted between RAM and disk.

If you try to power on a virtual machine and there is not enough memory available, GSX Server displays a warning message. The message shows how much memory the virtual machine is configured to use and how much memory is available. You can try to power on the virtual machine using the available memory by clicking OK. If you do not want to power on the virtual machine, click Cancel.