6.0

Estimating Memory Requirements for Virtual Machine Desktops
RAM costs more for servers than it does for PCs. Because the cost of RAM is a high percentage of overall
server hardware costs and total storage capacity needed, determining the correct memory allocation is
crucial to planning your desktop deployment.
If the RAM allocation is too low, storage I/O can be negatively affected because too much Windows paging
occurs. If the RAM allocation is too high, storage capacity can be negatively affected because the paging file
in the guest operating system and the swap and suspend files for each virtual machine grow too large.
RAM Sizing Impact on Performance
When allocating RAM, avoid choosing an overly conservative setting. Take the following considerations
into account:
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Insufficient RAM allocations can cause excessive Windows paging, which can generate I/O that causes
significant performance degradations and increases storage I/O load.
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VMware ESXi supports sophisticated memory resource management algorithms such as transparent
page sharing and memory ballooning, which can significantly reduce the physical RAM needed to
support a given guest RAM allocation. For example, even though 2GB might be allocated to a virtual
desktop, only a fraction of that number is consumed in physical RAM.
NOTE Transparent page sharing increases total consolidation ratios by reducing the total host memory
consumed by all desktops. Over a period of time, for desktop images running on the same host, the
transparent page sharing (TPS) feature collapses common memory blocks. The benefits of TPS accrue
over time and depend on the number of common memory blocks. For Windows 7, the expected TPS
ratio is between 20 and 40 percent.
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Because virtual desktop performance is sensitive to response times, on the ESXi host, set nonzero values
for RAM reservation settings. Reserving some RAM guarantees that idle but in-use desktops are never
completely swapped out to disk. It can also reduce storage space consumed by ESXi swap files.
However, higher reservation settings affect your ability to overcommit memory on an ESXi host and
might affect VMotion maintenance operations.
RAM Sizing Impact on Storage
The amount of RAM that you allocate to a virtual machine is directly related to the size of the certain files
that the virtual machine uses. To access the files in the following list, use the Windows guest operating
system to locate the Windows page and hibernate files, and use the ESXi host's file system to locate the ESXi
swap and suspend files.
Windows page file
By default, this file is sized at 150 percent of guest RAM. This file, which is
by default located at C:\pagefile.sys, causes thin-provisioned storage to
grow because it is accessed frequently. On linked-clone virtual machines, the
page file and temporary files can be redirected to a separate virtual disk that
is deleted when the virtual machines are powered off. Disposable page-file
redirection saves storage, slowing the growth of linked clones and also can
improve performance. Although you can adjust the size from within
Windows, doing so might have a negative effect on application performance.
Windows hibernate file
for laptops
This file can equal 100 percent of guest RAM. You can safely delete this file
because it is not needed in View deployments.
Chapter 4 Architecture Design Elements and Planning Guidelines for Remote Desktop Deployments
VMware, Inc. 43