VMware GSX 3 - Manual (Page 189)

C H A P T E R 6 Performance Tuning and the VMware GSX Server Host
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When you want to use a CD-ROM in the virtual machine, go to the VM >
Removable Devices menu and connect the CD-ROM drive.
Disk Options
The various disk options (SCSI versus IDE) and types (virtual or physical) affect
performance in a number of ways.
Inside a virtual machine, SCSI disks and IDE disks that use direct memory access (DMA)
have approximately the same performance. However, IDE disks can be very slow in a
guest operating system that either cannot use or is not set to use DMA.
The easiest way to configure a Linux guest to use DMA for IDE drive access is to install
VMware Tools (VM > Install VMware Tools). Among other things, the installation
process automatically sets IDE virtual drives to use DMA.
In Windows Server 2003, Windows XP and Windows 2000, DMA access is enabled by
default. In other Windows guest operating systems, the method for changing the
setting varies with the operating system. See the following technical notes in the
VMware GSX Server Virtual Machine Guide for details.
• Disk Performance in Windows NT Guests on Multiprocessor Hosts
• Windows 95 and Windows 98 Guest Operating System Performance Tips
When a snapshot exists, virtual disks often have very good performance for random or
nonsequential access. But they can potentially become so fragmented that
performance is affected. In order to defragment the disk, you must first remove the
snapshot (Snapshot > Remove Snapshot).
When no snapshot exists, physical disks and preallocated virtual disks both use flat
files that mimic the sequential and random access performance of the underlying
disk. When a snapshot exists and you have made changes since powering on the
virtual machine, any access to those changed files performs at a level similar to the
performance of a virtual disk that does not have all space allocated in advance. If you
remove the snapshot, performance is again similar to that of the underlying disk.
Overall, if no snapshot exists and you are using physical disks or preallocated virtual
disks, you see somewhat better performance than that provided by other
configurations.
Disk writes may be slower for virtual disks that do not have all space allocated in
advance. However, you can improve performance for these disks by defragmenting
them from the virtual machine settings editor. Choose VM > Settings, select the disk
you want to defragment, then click Defragment.