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RE: availability and resiliency

  • From: Roeland M.J. Meyer
  • Date: Fri Sep 29 14:34:58 2000

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.
-----Original Message-----
From: Leo Nelson []
Sent: Friday, September 29, 2000 10:08 AM
To: ''; Andrew Bangs
Subject: RE: availability and resiliency

Pardon my ignorance, but what the heck does a "9" refer too? Is it a UPS, rack, floor space, circuit...

Thanks in advance

-----Original Message-----
From: Lionel Lauer []
Sent: Friday, September 29, 2000 10:54 AM
To: Andrew Bangs
Subject: Re: availability and resiliency

On Fri, 29 Sep 2000 17:39:04 +0100, Andrew Bangs <>

>On Thu, Sep 28, 2000 at 02:39:40PM -0600, Irwin Lazar wrote:
>> Hi all,
>> Does anyone know if a template exists for what it takes to provide 5 9's of
>> availability, 4 9's etc.for Internet data centers?  Specifically I'm looking
>> for something that would say "if you want 5 9's of availability, here's what
>> you need to do", and so on.
>For 5 9s you need:
>1) Lots of money
>2) Lots of clue
>3) Lots of luck
>4) Lots of balls
>You can do 4 9s with any 3 of the above.

Too true.

But you forgot to include 'halfway-clued management' - without that you
haven't got a hope in hell of even getting three 9's. ;)

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