How to increase web server uptime with DNS failover

> viza wrote:
>>Guy Macon <http://www.GuyMacon.com/> wrote:
>>> How to increase web server uptime with DNS failover:
>>> Two servers with 99% uptime will also be down at the same time one day
>>> out of ten thousand. Three at 99% will be down at the same time one
>>> day out of a million.
>>...as long as they run different operating systems

>>and different servers

>>and host different websites,
>
> Nope. two servers in different places won't go down at the same time
> just because the are serving the same HTML.

>Guy Macon <http://www.GuyMacon.com/> wrote:
>> viza wrote:
>>>Guy Macon <http://www.GuyMacon.com/> wrote:
>>>> How to increase web server uptime with DNS failover:
>>>> Two servers with 99% uptime will also be down at the same time one day
>>>> out of ten thousand. Three at 99% will be down at the same time one
>>>> day out of a million.
>>>...as long as they run different operating systems
>>>and different servers
>>>and host different websites,
>>
>> Nope. two servers in different places won't go down at the same time
>> just because the are serving the same HTML.
>Just as an example: consider a site that goes down because of a
>vulnerability in some bloated php cms. If your two machines are running
>the same cgi scripts then they are more likely to both go down.
>Redundancy does increase uptime, but to claim to be able to calculate it
>quantitatively without in depth analysis of the likelihood and
>independence of every possible source of downtime is like picking numbers
>out of a hat. In case you don't have a degree in statistic and several
>years worth of logs for a few thousand servers then you are just making
>numbers up out of nothing.
>Your advice is worthwhile and may even be helpful to people, but your
>numbers are just a fantasy.

<http://www.GuyMacon.com/>

> viza wrote:
>>Guy Macon wrote:
>>> viza wrote:
>>>> Guy Macon wrote:
>>>>> How to increase web server uptime with DNS failover:
>>>>> Two servers with 99% uptime will also be down at the same time one
>>>>> day out of ten thousand. Three at 99% will be down at the same time
>>>>> one day out of a million.
>>>>...as long as they run different operating systems
>>>>and different servers
>>>>and host different websites,
>>>
>>> Nope. two servers in different places won't go down at the same time
>>> just because the are serving the same HTML.
>>Just as an example: consider a site that goes down because of a
>>vulnerability in some bloated php cms. If your two machines are running
>>the same cgi scripts then they are more likely to both go down.
>>Redundancy does increase uptime, but to claim to be able to calculate it
>>quantitatively without in depth analysis of the likelihood and
>>independence of every possible source of downtime is like picking
>>numbers out of a hat. In case you don't have a degree in statistic and
>>several years worth of logs for a few thousand servers then you are just
>>making numbers up out of nothing.
>>Your advice is worthwhile and may even be helpful to people, but your
>>numbers are just a fantasy.
>
> Your example of a CGI script bringing down multiple servers would also
> bring down a single server,and thus does not enter into a calculation
> for increasing web server uptime with DNS failover.

> If we were trying to predict absolute reliability, we would have to
> account for all such factors. If, as is the case here, we are
> calculating reliability increase due to using multiple servers with DNS
> failover, all such factors can be ignored.

> On Sun, 26 Oct 2008 17:11:35 +0000, Guy Macon wrote:
>
>> How to increase web server uptime with DNS failover:
>
>> Two servers with 99% uptime will also be down at the same time one
>> day
>> out of ten thousand. Three at 99% will be down at the same time one
>> day out of a million.
>
> ...as long as they run different operating systems and different
> servers and host different websites, and are on different internets
> and are independent in every other conceivable way.