The Gartner Group, a market research firm, has recently completed an unpublished study in which it compared the total number of hours of downtime of UNIX and NT machines in 100 corporations. It found that the average UNIX-based server experiences 23.49 hours of downtime per year; the average NT-based server experiences 89.98 hours of downtime per year. Assuming a library is open 4,000 hours per year, a UNIX server would be down less than 1 percent of the time (0.60%) and an NT server 2.25 percent of the time.
Our own experience is that the downtime of an NT server can be reduced to 1 percent by installing two servers in a cluster, with each capable of handling the entire workload. To do this is not prohibitively expensive from a hardware standpoint, but it can be costly if each machine requires a separate license from the software vendor. The difficulty of negotiating reasonable license terms for NT has led a number of libraries to decide on UNIX as the operating system, rather than pursuing a special licensing agreement for an NT cluster. It is hoped that this situation will change as vendors gain more experience with NT.