The following sections describe ways you can improve the performance of GSX Server on both Windows and Linux hosts.

Make certain you select the correct guest operating system for each of your virtual machines. To check the guest operating system setting, choose VM > Settings > Options > General.

GSX Server optimizes certain internal configurations on the basis of this selection. For this reason, it is important to set the guest operating system correctly. The optimizations can greatly aid the operating system they target, but they may cause significant performance degradation if there is a mismatch between the selection and the operating system actually running in the virtual machine. (Selecting the wrong guest operating system should not cause a virtual machine to run incorrectly, but it may degrade the virtual machine's performance.)

Make sure to choose a reasonable amount of memory for your virtual machine. Many modern operating systems have a growing need for memory, so assigning a generous amount is beneficial for the best virtual machine performance.

The same holds true for the host operating system, especially a Windows host.

The New Virtual Machine Wizard automatically selects a reasonable starting point for the virtual machine's memory, but you may be able to improve performance by adjusting the settings in the virtual machine settings editor (choose VM > Settings > Memory).

If you plan to run one virtual machine at a time most of the time, a good starting point is to give the virtual machine half the memory available on the host.

Adjusting the reserved memory settings may also help. Choose Host > Settings > Memory.

For additional information, see Understanding Memory Usage.

You can configure each virtual machine to run in one of two modes — normal mode and a mode that provides extra debugging information. The debugging mode is slower than normal mode.

For normal use, make sure the virtual machine is not running in debugging mode. Choose VM > Settings > Options and select Advanced. Under Settings, make sure the Run with debugging information check box is cleared.

Some operating systems — including Windows NT and Windows 98 — poll the CD-ROM drive every second or so to see whether a disc is present. (Doing this allows them to run autorun programs.) This polling can cause GSX Server to connect to the host CD-ROM drive, which can make the CD-ROM drive spin up while the virtual machine appears to pause.

If you have a CD-ROM drive that takes especially long to spin up, there are two ways you can eliminate these pauses.

For information on finding TweakUI and installing it in your guest operating system, go to www.microsoft.com and search for TweakUI. Specific instructions depend on your operating system.

To make this change, go to VM > Settings. Select the DVD/CD-ROM item in the Device list. Then clear the Connect at Power On check box.

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.

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 for details.

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.

Whenever possible, do not use disks that are on remote machines and accessed over the network unless you have a very fast network. If you must run disks remotely, choose VM > Settings > Options, select General and set the Working directory to a directory on your local hard disk. Then take a snapshot. After you take the snapshot, changes you make are stored locally in the working directory.

If you do not need to use snapshots, it is best to run your virtual machine without a snapshot. This provides best performance. To be sure a virtual machine has no snapshot, choose Snapshot > Remove Snapshot.

You may notice that whenever you try to install or start a particular program in a virtual machine, the program seems to hang, crash or complain that it is running under a debugger. VMware has seen this problem with a few programs, including the installer for the Japanese version of Trend Micro Virus Buster, the FoxPro database, the NetWare client in Windows 98, Mathcad, The Sims and Civilization III.

You can work around this problem by using a special setting called disabling acceleration. Frequently, the problem occurs only during installation or early in the program's execution; in that case you should turn acceleration back on after getting past the problem. Follow these steps:

1. Power on the virtual machine.

2. Before running or installing the program that was encountering problems, disable acceleration.

Choose VM > Settings > Advanced and check Disable acceleration.

3. Click OK to save the change and close the virtual machine settings editor.

4. Start the program or run the installer.

5. After you pass the point where the program was encountering problems, return to the virtual machine settings editor and remove the check beside Disable acceleration. You may be able to run the program with acceleration after it is started or installed.