Even while Second Life struggles with its explosive growth and new popularity, the developer, Linden Lab, is redesigning the service to handle hundreds of times as much traffic as it's dealing with today.

Second Life is seeing 20% monthly growth in the number of concurrent users, going from a then-record 18,000 users logged in at once in December to a record 36,000 on Sunday. The maximum capacity of the current network is 100,000 simultaneous users, says Joe Miller, VP of platform and technology development for Linden Lab.

Rapid growth has led to problems for Second Life users, who often complain about slow response times during peak periods. Users also see problems with screen graphics, moving around between regions of the virtual world, and discrepancies in transactions involving the in-world currency, called "Linden Dollars." Search results are becoming out of date quickly.

To help deal with current and future growth, Linden Lab is looking to redesign the network. Linden Lab is looking to increase capacity from 100,000 simultaneous sessions to tens of millions of simultaneous logins. Linden Lab also needs to manage that upgrade without interrupting service to existing users.

"We're swapping engines out at 40,000 feet while still flying," Miller says.

Second Life runs on 2,000 Intel and Advanced Micro Devices servers in two co-location facilities in San Francisco and Dallas. The company has a commitment to open source, with servers running Debian Linux and the MySQL database. Linden Lab chose Debian Linux because the software is suited to scaling massively with a small IT staff, say Linden Lab CTO Cory Ondrejka. MySQL allows the server farms to scale horizontally, by adding large numbers of low-power servers as needed, rather than vertically, which would have required Second Life to run on a few, powerful systems, Miller says.

The architecture of Second Life is based on the illusion of a virtual world, built out of virtual land. Each geographic area in Second Life runs on a single instantiation of server software, called a simulator or "sim." And each sim runs on a separate processor on a server. So, when an avatar walks or flies or swims or drives from one sim to an adjacent sim, it's actually moving from one processor or server to another.

Users in Second Life -- who are known as "residents" -- buy virtual land to house their buildings and virtual furniture, vehicles, and other stuff. In real-world terms, what the residents are doing is leasing software capacity on a server. Nearly everything in Second Life, all the buildings, clothes, vehicles, furniture, avatars, and other objects, was built by residents and operates on resident-owned land.