NW -
Stability refers to the way in which the system behaves over time - basically that what worked yesterday will work today - in particular, through different revisions of the OS. It is related to availability, which is the amount of time the system is actually usable. Mature operating systems like MVS and OpenVMS have had nearly all of the potential interaction pitfalls worked out, and so they are very predictable. That stability is usually achieved at the expense of growth in capability, since changes to the OS tend to create opportunities for failures. IT shops develop practices to achieve acceptable availability, but when the underlying OS changes in ways that cause new problems to appear, they have to start all over again. This is what is meant by lack of stability.
A well-managed MVS system can have availability in excess of 99.9% (about 8 hours unavailable per year), and OpenVMS routinely does 99.99%(45 minutes per year). Tandem systems can deliver 99.9999% (less than 30 seconds per year unscheduled downtime). DEC Unix and Solaris are between 99.9 and 99.99 depending on configuration. NT, by comparison, is about .95 (about 1 week per year), even in a very well managed installation, because of the need to take the system down when adding hardware or changing software.
A younger OS like NT is building features very rapidly, and thus the environment is constantly changing. An IT crew who learned to arrange things to achieve .95 on NT version 4 with service pack 3 may need features of service pack 4, but they know that MS testing is not geared to stability, but rather to capability of features. So the IT shop must do its own testing, identify problems, and develop procedures to control them or work around them.
The various flavors of Unix are not as stable as a proprietary OS but they are more stable than NT, as they come from a common and relatively mature base. So IT shops spend less money maintaining and testing Unix installations. For large systems that is an important value. For many smaller systems, such as departmental servers, the advantages of NT outweigh the inconveniences.
NT will certainly have to solve this problem if it is to extend into the upper reaches of the enterprise. |
