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VMware ESX Server Administration Guide
quickly enough to satisfy current system demands. Standard demand paging
techniques swap pages back in when the virtual machine needs them.
The vmmemctl approach is used whenever possible for optimum performance.
swapping is a reliable mechanism of last resort that the system uses to reclaim
memory only when necessary.
Swap Space and Guest Operating Systems
If you choose to overcommit memory with ESX Server, then you need to be sure your
guest operating systems have sufficient swap space. This swap space must be greater
than or equal to the difference between the virtual machine’s maximum and
minimum sizes.
Caution: If memory is overcommitted, and the guest operating system is configured
with insufficient swap space, the guest operating system in the virtual machine may
fail.
To prevent virtual machine failure, increase the swap size in your virtual machines:
• Windows guest operating systems — Windows operating systems refer to their
swap space as “paging files.” Some Windows operating systems automatically try
to increase the size of paging files, provided there is sufficient free disk space.
For more information, refer to your Windows documentation or search the
Windows help files for “paging files.” Follow the instructions for changing the
size of the virtual memory paging file.
• Linux guest operating system — Linux operating systems refer to their swap
space as “swap files.” For information on increasing swap files, refer to the
mkswap (sets up a Linux swap area) and swapon (enables devices and files for
paging and swapping) man pages found in your Linux guest operating system.
Guest operating systems with large memory and small virtual disks (for example, a
virtual machine with 3.6GB RAM and a 2 GB virtual disk) are more susceptible to this
problem.
Sharing Memory Across Virtual Machines
Many ESX Server workloads present opportunities for sharing memory across virtual
machines. For example, several virtual machines may be running instances of the
same guest operating system, have the same applications or components loaded, or
contain common data. In such cases, ESX Server uses a proprietary transparent page
sharing technique to securely eliminate redundant copies of memory pages. With
memory sharing, a workload running in virtual machines often consumes less