Setting Memory Limits for All Virtual Machines

Select: Host > Settings > Memory

VMware Server allows you to set the memory size of each virtual machine and the amount of the host computer's memory that can be used for virtual machines. A third control governs the extent to which you want to allow the host operating system's memory manager to swap virtual machines out of physical RAM.

By adjusting these settings, you can affect both virtual machine and overall system performance.

Memory Use on the Host

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, VMware 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 VMware Server processes cannot exceed the amount of physical memory on the host minus some memory that must be kept available for the host. However, 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.

If you want the most memory out of your virtual machine and are pushing close to the supported limits, you should expect that if you run the virtual machine for a while, power it off and change settings in the virtual machine settings editor (especially increasing the virtual machine's memory size), then try to power it back on, the virtual machine may not power on. If this happens, close the console in which you tried opening the virtual machine, and open a new console. The virtual machine should power on.

Specifying How Much Memory is Used by All Running Virtual Machines

You can set is the amount of host RAM that VMware Server is allowed to reserve for all running virtual machines. To set this parameter, choose Host > Settings > Memory. You must be user to change these settings.

The reserved memory setting specifies a maximum amount of host RAM that VMware Server is allowed to use. But this memory is not allocated in advance. Even if multiple virtual machines are running at the same time, VMware 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 Machine Overhead

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

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 dynamically 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 control — Host > Settings > Memory. If you want VMware 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 if other applications are run on the host. If you set this value too low, virtual machines may perform very poorly and you cannot run as many virtual machines at once.

Using Additional Memory

By default, VMware Server limits the number of virtual machines that can run at once based on the amount of memory specified in the application settings. This 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, 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 the option under Additional memory.

Select one of the following options:

Fit all virtual machine memory into reserved host RAM — Strictly apply the reserved memory limit set in the top of the panel. 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, VMware Server displays a warning message. The message shows how much memory the virtual machine is configured to use and how much memory is available. To try to power on the virtual machine using the available memory, click OK. If you do not want to power on the virtual machine, click Cancel.