Web Capacity Planning: How to Plan for Server Growth

Dave Zwieback

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There is surprisingly little relevant literature on capacity planning, and having the latest and the fastest equipment doesn't always guarantee the best performance, nor is always economically feasible. As a result, sys admins rely on a mixture of experience and "magic" in right-sizing their systems. In this article, I provide some practical advice on selecting a system and planning for growth, as well as getting the most of your system for Web server purposes.

HTTP over TCP/IP

Web clients (browsers) and servers communicate via the Hyper Text Transfer Protocol (HTTP). A typical Web client makes a connection to a Web server, requests and receives one HTTP object (e.g., an HTML file or a JPEG image) or an error message, then closes the connection. (For an example of what actually happens, telnet to port 80 of www.samag.com and type "GET /" followed by a return.) A typical HTML page contains several references to other HTTP objects (like in-line images and sounds), so the client makes separate connections for each of the remaining objects before the page is properly displayed on the screen. (Netscape Navigator by default makes up to four connections to the server in rapid succession, so several objects may be loading "simultaneously".)

A relatively small amount of data is transferred over each such connection, and it is not uncommon for a Web server to receive millions of connections a day. Thus, current out-of-the-box TCP/IP implementations may not be the best match for the HTTP world, since they are usually optimized for a few long-lived connections with substantial amounts of data transferred.