VMware GSX Server 2.5.2

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To adjust the amount of memory reserved for all running virtual machines, on a Windows host in the Preferences dialog box, click the Memory tab.

On a Linux host, choose Settings > Reserved Memory. A dialog box allows you to adjust the amount of memory reserved to all running virtual machines.

Virtual machines require relatively large amounts of memory to operate with reasonable performance. This memory comes out of the same pool of memory used by all applications on the host machine. The memory used by GSX Server includes the memory for the virtual machine itself as well as overhead memory associated with running a virtual machine. The virtual machine's memory size is specified in the virtual machine configuration.

GSX Server reserves the host's physical memory to improve virtual machine performance. This is done by locking physical memory. The amount of physical memory that is actually locked for a particular virtual machine varies dynamically as it runs. If several virtual machines run simultaneously, they work together to manage the locked memory.

To provide good overall system performance, GSX Server limits the amount of the host's memory that can be locked by all virtual machines together. Any reserved memory used by virtual machines is not available to the other applications or to the host operating system. GSX Server uses the reserved memory only if it determines that a virtual machine needs reserved memory to have reasonable performance. Even if multiple virtual machines are running at the same time, GSX Server may be using only a fraction of the reserved memory, thus allowing any unused reserved memory to be used by other applications. Within this reserved memory pool an individual virtual machine can use at most the amount of memory configured 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.

GSX Server allows you to set the maximum amount of memory to reserve. The recommended amount of memory to reserve for GSX Server applications is 50% of the host's physical memory. If you determine that you want GSX Server to reserve more or less physical memory, you can change this amount by changing the value in Reserved size (MB). Changing the amount of reserved memory is recommended only for advanced users because it can have an adverse impact on host or virtual machine performance. Selecting to reserve too much physical memory can cause the host to thrash, or even hang, if other applications are run on the host. Selecting to reserve too little physical memory can cause virtual machines to perform very poorly and also limit the number of virtual machines that can be run.

By default, a virtual machine performs two checks to ensure that enough free memory is left to the host machine to continue to operate efficiently. First, at power on, it checks that the total amount of memory allocated to all running virtual machines does not exceed the total physical memory of the host. Second, the virtual machine periodically checks that the reserved memory allocated to currently running virtual machines does not exceed the total amount of reserved memory. If the first check fails, the virtual machine does not power on; if the second check fails, GSX Server attempts to compensate by reducing the virtual machine's memory use.

If you want to disable this check on memory limits, see Disabling Memory Checks for Virtual Machines.

For more information about memory and virtual machines, see Understanding Memory.