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== Host-wide Memory Settings ==
Inherent in the concept of virtualization is the fact that a virtual machine must share the physical memory of the host computer system with both with other virtual machines and the host operating system. When each VMware Server virtual machine is created, the amount of memory allocated to the particular machine is specified during the initial configuration process and may subsequently be modified using the management tools provided. As such, the host is responsible for allocating memory and will allocate memory based on the requirements of the virtual machines in conjunction with the needs of the host operating system and any other host based processes that happen to be running at any particular time. The key issue with this scenario, however, is that VMware Server is free to request as much memory as necessary to run virtual machines. Without some form of cap on the amount of memory that can requested, there is a danger that so much memory will be requested by virtual machines that there will be insufficient memory for the host operating system and other applications to function, thereby degrading the overall performance of the host, and inevitably also the virtual machines.
In order to address this potential problem, VMware Server allows the configuration of a host-wide VMware Server ''reserved memory limit''. This limit specifies the maximum amount of memory VMware Server can request for running virtual machines. Arriving at the correct setting is something of a balancing act and depends on both the physical memory capacity of the host and the requirements of the virtual machines. When more host memory is requested for virtual machines than is allowed by the limit the memory the virtual machines will perform slowly or fail to power on. If the reserve limit is set too high, there is the danger that the host performance will be impacted if virtual machines use the entire reserved memory.
When the memory limit is higher than the current usage, however, the unused portion of the reservation is still available for use by the host and other applications. As such, it is important to understand that this setting is in no way a guarantee of the amount of memory that is available to virtual machines (since the memory may already be in use by other application by the time VMware requests it for a newly started virtual machine), but rather a limit on the amount of memory that can be requested by VMware.
The amount of memory that may be used by VMware Server for virtual machines is configured using the VI Web Access management interface (details of which are covered in the [[A Guided Tour of the VMware Server 2.0 Infrastructure Web Access Interface]] chapter of this book).
To alter the reserved memory limit, log into the VI Web Access interface as a user with administrative privileges, select the host from the ''Inventory'' panel, and click on the ''Edit Host Settings'' link in the ''Commands'' panel to invoke the following dialog:
== Memory Swapping Configuration ==
In an ideal world, computers would have enough physical memory to run as many applications concurrently as any user could ever need. Unfortunately, that day has yet to come and most computer systems quickly use up all the available memory. In this scenario, the operating system performs a task called ''swapping'' whereby data held in memory is written to a hard disk to make room for other data. When the data written to the swap space on the disk is needed, the operating system writes some other memory based data to the disk to make room, and then reads previously swapped data back into memory. Whilst this seems like an elegant solution to the problem of limited memory availability, it has the downside that disk drives work much more slowly than random access memory (RAM). the The more an operating system has to swap memory, therefore, the slower it will run.
As the use of memory and swap space has have a significant impact on the performance of both the host system and virtual machines, VMware Server provides a number of configuration options in this area. To change host memory swapping settings, log into VI Web Access as a user with administrative privileges, select the host from the ''Inventory'' panel and click on ''Edit Host Settings'' in the ''Commands'' panel. The relevant settings are listed in the ''Additional Memory'' section of the dialog as illustrated in the following panel:
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Restricts virtual machines to running in memory only. Virtual machines are not swapped to disk thereby providing maximum performance. Host The host system must have sufficient memory for the host operating system, all virtual machines and other host based applications.
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<td>Allow some virtual machine memory to be swapped</td>
<td>Allows some, but not all, of the memory used by virtual machines to be swapped to disk by the host operating system. Results in some performace performance degradation as compared to running solely in memory, but allows a greater number of virtual machines to be ruin run concurrently.</td>
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To enable the automatic startup and shutdown of virtual machines when the host power state changes, select the box next to ''Allow virtual machines to start and stop automatically with the system''.
To impose a delay between started starting or stopping each virtual machine, enter the required timeout durations in the ''Default Startup Delay'' and ''Default Shutdown Delay'' fields. In the case of the host shutting down, it is also possible to select the shutdown mechanism used to power off any currently running virtual machines using the ''Shutdown Action'' menu.
Since the purpose of the startup delay is to ensure that each virtual machine is fully booted before starting the next virtual machine, an interesting option is available for guests operating systems which have VMware Tools installed. Since VMware Tools are amongst the first processes to be started once a guest operating system has completed its startup process, VMware Server can be instructed to ignore any time remaining on the timeout delay if the presence of VMware Tools running on the guest is detected. In other words, if the startup delay is configured for 120 seconds and the guest actually starts up (and launches VMware Tools) in 90 seconds, VMware can immediately move on to starting the next virtual machine with waiting unnecessarily for the remaining 30 seconds of the delay time. To activate this option, make sure that the ''Start next VM immediately if the VMware Tools start'' option is selected.
The lower section of the dialog presents the current startup order for the virtual machines contained with the hosts inventory. Three categories of list are provided consisting of virtual machines which are to be started in a specific order, virtual machine that may be started in any order, and virtual machines which are to started manually. To move a virtual machine from one category to another, select it from the list and use the ''Move Up'' and ''Move Down'' buttons. When the virtual machine reaches the ''Specified Order'' list, the up and down arrows control the position of virtual machine in the startup order. To remove a virtual machine from the startup order list, press the down button until it moves out of the Specified Order list and into the next category.
To modify the settings for a specific virtual machine, select it from the list and click the ''Edit'' button. The resulting dialog provides the option to override the host wide defaults and set specific startup and shutdown settings for the selected virtual machine.
