3.2.5 System Down Time Calculations
The following table illustrates the probability that on any given day the messaging service will be unavailable due to system failure. These calculations assume that on average, each server goes down for one day every three months due to either a system crash or server hang, and that each storage device goes down one day every 12 months. These calculations also ignore the small probability of both nodes being down simultaneously.
Table 3–2 HA Down Probabilities|
Model |
Server Down Time Probability |
|
Single server (no high availability) |
Pr(down) = (4 days of system down + 1 day of storage down)/365 = 1.37% |
|
Asymmetric |
Pr(down) = (0 days of system down + 1 day of storage down)/365 = 0.27% |
|
Symmetric |
Pr(down) = (0 days of system down + 0 days of storage down)/365 = (near 0) |
|
N + 1 Asymmetric |
Pr(down) = (5 hours of system down + 1 day of storage down)/(365xN) = 0.27%/N |
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