What does fault-tolerant or high-availability really mean? Likewise, if a system says it has five nines of availability what does that really mean?
|
System |
Downtime |
Downtime |
Downtime |
Availability |
Availability |
|
unmanaged |
36.5 days |
72 hours |
16.8 hours |
90.% |
1 nine |
|
managed |
3.65 days |
7.20 hours |
1.68 hours |
99.% |
2 nines |
|
well |
8.76 hours |
43.2 mins |
10.1 mins |
99.9% |
3 nines |
|
fault |
52.6 mins |
4.32 mins |
1.01 mins |
99.99% |
4 nines |
|
high |
5.26 mins |
25.9 secs |
6.05 secs |
99.999% |
5 nines |
|
very high |
31.5 secs |
2.59 secs |
0.605 secs |
99.9999% |
6 nines |
|
ultra |
3.1 secs |
.25 secs |
.06 secs |
99.99999% |
7 nines |
References
- Gray, J. & Sieworek, D. (1991)IEEE Computer, Vol. 39, No 8, pp 39-48
http://research.microsoft.com/en-us/um/people/gray/papers/ieee_HA_Swieorick.pdf - Wikipedia
http://en.wikipedia.org/wiki/High_availability