Host operating systems do not behave well when they run low on free memory for their own use. When a Windows or Linux host operating system does not have enough RAM for its own use, it thrashes — constantly swaps parts of itself between RAM and its paging file on disk. To help guard against virtual machines causing the host to thrash, GSX Server enforces a limit on the total amount of RAM that may be consumed by virtual machines.

In general, the sum of the memories of all currently running virtual machines plus overhead for the GSX Server processes cannot exceed the amount of physical memory on the host minus some memory that must be kept available for the host. For more information, see Using Additional Memory.

Some memory must be kept available on the host to ensure the host is able to operate properly while virtual machines are running. The amount of memory reserved for the host depends on the host operating system and the size of the host computer's memory.