One of the main features of ISA Server is its ability to cache Web pages so that clients can be served from the cache instead of Internet. In order words, when a client requests a Web page, ISA Server, according to the configured rules, retrieves that page from the Web, caches it, and returns the page to the client. When another client requests the same page, it can now be directly returned to the client from the cache, according to the TTL value. This caching function accounts for ISA's "acceleration feature. After all, ISA Server can more quickly serve a client from its disk cache than from the Internet
Cached items are stored on the ISA Server's local hard drive. Through hierarchical and distributed caching, items stored on other ISA Servers can also be retrieved and provided to the client. However, the process of pulling information from a hard disk can also take time, and in an effort to make ISA Server provide the fastest client service possible, RAM caching is also available for use.
RAM caching simply means that ISA Server can store the most popular objects both in its disk cache and in RAM. When a client requests an object stored in RAM, it can be immediately returned to the client without having to be read from the disk, which makes service to clients faster. By default objects that are smaller than 12800 bytes are stored in RAM and on the ISA Server's disk cache. Objects larger than this are only stored on the disk. By default ISA Server is configured to use 50% of its free memory to store cached objects. If you access the Cache Properties pages, Advanced tab, as shown in Figure 1, you can see the RAM caching option at the bottom of the window and the percentage configuration dialog box.
Curt Simmons is the Author of 'Microsoft Internet Security and Acceleration Server 2000 Study Guide : Exam 70-227 ( Certification Study Guides)'
Curt Simmons is also the Author of 'Microsoft ISA Configuration and Administration '
As I mentioned, the 50% value means that ISA Server will use 50% of the free memory available at any given time for caching. Depending on the applications and services you are running at the moment, the actual amount of free RAM will vary, so exactly how much RAM is available to ISA Server to use for caching will vary from system to system. Obviously, the more RAM the server has and the less additional server services it provides to the clients, more RAM will be available for caching.
Of course, you can change the 50% default value if you so choose. Before raising this value, however, you might consider using Performance monitor and checking out the memory cache values so you can see how ISA Server may be able to handle the increase. Also, if you increase the value, I recommend that you do so on an incremental basis so you check the performance of ISA Server as you increase the amount of RAM that you want to use for caching. Of course, if you do not want RAM to be used for caching at all, simply set this value to 0.
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Curt Simmons, MCSE, MCT, CTT, is a technology author and trainer from Dallas. Visit Curt on the Internet at http://curtsimmons.hypermart.net.