Specifications

SX-200 ICP General Information Guide
164
Defining terms
The term five-nines refers to availability more than reliability, although reliability is integral to
availability. Availability is determined by two basic factors; Mean time Between Failures (MTBF),
sometimes referred to as Mean Time Between Outages (MTBO), and Mean Time to Repair
(MTTR). Both of these are commonly measured in hours. Availability is described by the
following equation:
Availability = MTBF/(MTBF + MTTR) = .9xxxxx
where:
Mean Time Between Failure (MTBF): term used to estimate the reliability of a product’s
hardware.
Mean Time To Repair (MTTR): the mean time to restore service rather than to repair a
component. MTTR includes the following five activities:
• Failure Detection
• Failure Notification
• Vendor/User Response
• Repair/Replacement
• Recovery/Restart/Reboot
MTBF provides a measure of a system’s reliability. However, over the system’s lifetime, this
metric does not necessarily identify everything you need to know. Your system could have 99%
availability and still suffer a disaster (one huge outage or many short outages) and still produce
the same availability. Although these metrics do not take the impact of outages into
consideration, they still provide a frame of reference.
The metrics for five-nines includes performance for the following system elements and
components:
• Hardware components (CPUs, NSUs, ASUs)
• Power supplies
• Any other hardware component that can cause a total failure
The calculation does not include the following items:
• Shutdown of the operating system software
• Loss of electrical power
• Network loss
• Time required for application software upgrades and fixes
• Time required for preventative maintenance
Note: The first four items in this list can be significantly reduced or eliminated by
redundant components and automatic reconfiguration.