Scaling for Terminal Services

Terminal services have changed drastically over the years . Early versions of terminal services had issues with multiple processors, they didn't have the ability to load balance, and they had no ability to leverage local client resources. Third-party add-ons to Terminal Services addressed these types of issues and increased the ability to scale Terminal Services into larger and larger deployments. The current version of Terminal Services in Windows 2003 has natively addressed these concerns and has proven its ability to scale to very large implementations .

Big Processors Versus Multi-Processors

Current processor technologies allow servers to perform incredible amounts of computing. Single servers can host literally hundreds of simultaneous users. Windows 2003 Terminal Services is able to scale performance nearly 1:1 with the addition of multiple processors up to four processors. Although Terminal Services can be run with more processors, benchmarking has shown that scaling beyond four processors results in greatly diminishing returns.

Memory, Memory, and More Memory

Terminal Services can be a fairly memory hungry beast . Exact memory requirements will vary dependant on the types of users accessing the system and the applications running on the system itself. A safe rule of thumb is 16MB per user if he is running specific applications and 32MB per user if he is running a desktop session. Memory for the system itself should be added to this value to determine the total memory needed for the terminal server. Without enough memory to support the users the individual user 's performance will suffer. Sufficient memory is needed to properly scale Terminal Services.

Terminal Service Farms

To support very large numbers of Terminal Services users it is necessary to go beyond one or two Terminal Servers and into a full Terminal Service farm. Because of the unique needs of Terminal services there are a few components that are critical to the success of the farm. Some applications require the tracking of a session state. A Web server that is load-balanced might need the user to return to a specific Web server that was tracking the user's actions in order to make an application work properly. Terminal Services takes this concept much further. Terminal Services give the user the capability to disconnect from the session but have the applications continue to run. For this reason it is an absolute necessity for a user to reconnect to his original session. This is accomplished via the Session Directory.

By having Terminal Servers join the Session Directory, the Session Directory Servers will track which users are on which Terminal Servers. This allows users who are intentionally or unintentionally disconnected to return to their original session when they reconnect. Because of the importance of this role, it is recommended that the Session Directory be run on a cluster.

Because users of a Terminal Services farm can conceivably connect to any server in the farm it is important that their personal resources be reachable from any session. It is best to think of a user's terminal server session as a disposable resource. Nothing unique to that user should be stored on a Terminal Server. The easiest way to accomplish this is through the use of Terminal Server profiles and redirected folders. These folders should redirect to a central file store. This file store could potentially be used by hundreds of Terminal Servers and therefore thousands of Terminal Server users. Use of NAS with a clustered head is highly recommended for this role. To make sure that users don't store data locally to the session the session should be locked down via GPOs. Decisions on whether or not to allow users to connect to their local host are left to the individual administrator.

The responsible administrator will take the time to lock down the servers application by application to prevent users from altering resources that they shouldn't have access to.

Improving Scalability by Load Balancing Applications

Windows 2003 offers load balancing amongst Terminal Servers. For some environments, this isn't enough. Several third-party vendors have added the concept of Application Load Balancing to Terminal Services. This means that if a user wanted to run Word their request would reach an Application Load Balancer and it would check to see which server offering Word had the lowest load. This is the server that the user would be connected to. This allows an administrator to load balance all of her servers to a single name and IP address and not have to install all applications on all servers. This can be especially helpful when running applications that have specific local peripheral requirements.