Dependable Memory Hierarchy

Fast but undependable is not very attractive. One of the 8 Great Ideas in Computer Architecture for dependability is Redundancy.

Reliability is a measure of the continuous service accomplishment—or, equivalently, of the time to failure—from a reference point

We define 2 terms:

• mean time to failure (MTTF)
• annual failure rate (AFR)

When MTTF gets large it can be misleading, so we use AFR for better intuition.

$\text{Availabilitty}=\frac{MTTF}{MTTF+MTTR}$

• Where MTTR is the mean time to repair

There are three ways to improve MTTF:

1. Fault avoidance: Preventing fault occurrence by construction.
2. Fault tolerance: Using redundancy to allow the service to comply with the service specification despite faults occurring.
3. Fault forecasting: Predicting the presence and creation of faults, allowing the component to be replaced before it fails.