Windows 2003 Server is a more reliable server operating system than Linux, a research firm said Monday.

According to the Yankee Group's annual server reliability survey, only Unix-based operating systems such as HP-UX and Sun Solaris 10 beat Windows on uptime. Windows 2003 Server, in fact, led the popular Red Hat Enterprise Linux with nearly 20 percent more annual uptime.

On a broader note, said Yankee analyst Laura DiDio, the major server operating systems all have a "high degree of reliability," and have showed marked improvement in the last 3 to 5 years.

On average, individual enterprise Windows, Linux, and Unix servers experienced 3 to 5 failures per server per year in 2005, generating 10 to 19.5 hours of annual downtime for each server.

But standard Red Hat Enterprise Linux, and Linux distributions from "niche" open source vendors, are offline more and longer than either Windows or Unix competitors, the survey said. The reason: the scarcity of Linux and open source documentation.

The Yankee Group made a point of stressing that the survey was not sponsored or supported by any server OS maker.