"UNIX® machines run and run (and run). A desktop or portable
system can be left on for weeks, even months, and server uptime can
stretch to a year or more. Indeed, if you could preclude hardware
failures and Mother Nature, a UNIX system might run perennially.
Alas, hardware is imperfect, Mother Nature has a mind of her own,
and software suffers from bugs. The gear requires replacement.
Thunderstorms can and do happen. And systems require patches and
restarts. Much like death and taxes, downtime is a certainty.
"However, unlike death and taxes, you can minimize downtime.
Proactive and regular system maintenance and hot spares boost
availability, as does a robust data center replete with backup
power and redundant connections to the Internet. Thankfully, too,
most applications and libraries can be updated seamlessly and on
demand using tools such as rpm, Aptitude, and yum.
"Unfortunately, kernel updates—modifications to address
vulnerabilities and flaws in the core system software—aren't
so painless. A kernel update is very disruptive, requiring
scheduled downtime to temporarily halt all services on each and
every machine affected. Although such upkeep is necessary and
vital, keeping pace with kernel updates can nonetheless make
operations something akin to a yo-yo."