North American Network Operators Group|
Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical
RE: availability and resiliency
Title: RE: availability and resiliency
It refers to the percent uptime of a host or site. 99% is two "9"'s (2x9?) and 99.9% is three nines. Getting a single host to meet more than three nines (99.9%) can be a challenge ( <8.76 hours outage, per year), but can be more easily met with multiple hosts in a site. Four nines (99.99% uptime, <0.88 hours annual downtime) is extremely difficult for a single host, less difficult for internal data centers, and (given lots of $$$) a bit easier for a internet site (using multiply redundant hosts). Five nines (99.999%, <5.26 annual minutes down) is almost impossible for a single affordable host to meet. This is where we enter the world of High-Availability (H-A) systems. These are usually high transaction flow critical systems and are found in large corps, telcos, and reliable internet sites. At this time, only governments are willing to part with the required cash to build systems meeting six nines (99.9999%, <0.53 minutes annual downtime), or better (NASA, NORAD, US Space Command, etc). Usually, this is done using multiple site redundancy.
Hosts meeting three nines, or better, typically have redundant power supplies and integrated UPS, bootable RAID for the OS, redundant NICs, and SMP CPU configurations.