How to increase web server uptime with DNS failover

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

Guy Macon <http://www.GuyMacon.com/> wrote:
>How to increase web server uptime with DNS failover:
>For illustration, let's start with some really bad hosting...
>Find four free web hosts that are each 90% reliable
>-- in other words they each are down on average one
>day out of ten.  Use a failover DNS system with monitoring
>to switch servers if one fails.  On average:
> One server will be down one day out of ten
> Two servers will be down one day out of a hundred
> Three servers will be down one day out of a thousand
> All four servers -- and thus your website -- will be
> down at the same time one day  out of ten thousand.
>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.
>In addition, there will be a delay for some users before
>the new server is switched in.  One website measured this
>as follows:
>After  5 minutes,  3% of visitors see the new server.
>After 10 minutes, 20% of visitors see the new server.
>After 15 minutes, 37% of visitors see the new server.
>After 20 minutes, 59% of visitors see the new server.
>After 25 minutes, 69% of visitors see the new server.
>After 30 minutes, 76% of visitors see the new server.
>After 35 minutes, 80% of visitors see the new server.
>After 45 minutes, 90% of visitors see the new server.
>After 90 minutes, 95% of visitors see the new server.
>After 18 hours,   99% of visitors see the new server.
>With any number of servers at 90% uptime, this will
>happen on average every ten days. With any number of
>servers at 99% uptime, this will happen on average
>every hundred days.
>Needless to say, this works even better if you start
>with high-uptime paid servers.
>References:
>http://blog.pyromod.com/2007/09/viability-of-dns-failover.html
>http://www.webhostingtalk.com/showthread.php?tR4788
>http://www.dnsmadeeasy.com/s0306/prod/dnsfosm.html
>http://www.georedundanthosting.com/products/what-is-geographically-redundant-hosting.aspx
>http://edgedirector.com/app/back.htm
>http://www.webstrikesolutions.com/Page_ID.aspx?IDTXT6

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

> 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.

>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,

>and are on different internets

>and are independent in every other conceivable way.

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

> 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
>
> Nope. They can both be running on linux.  If every linux system
> goes down at the same time, having one of your copies on, say,
> solarus won't help you: the entire internet will be down.
>
>> and different servers
>
> I already specified that. Did you bother to read the post you are
> responding to?
>  
>> 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.
>
>> and are on different internets
>
> Now you are just being silly.
>
>> and are independent in every other conceivable way.
>
> That's not how the math behind redundancy works.  the two
> redundant systems have to not share a single point of
> failure.  Being "independent in every other conceivable
> way." is not a requirement.  My cellphone and my laser
> printer both share the attribute of having copper
> conductors and silicon microprocessors, (and thus are
> not " independent in every conceivable way" but those
> are not common points of failure.  A copper wire may
> fail, but all copper wires on earth are not going to
> fail at the same time.

> DNS failover can increase web server uptime.  That is a
> basig engineering calulation that has been tested many
> times of many systems.
>
>  "Arguing with anonymous strangers on the Internet is a sucker's
>  game because they almost always turn out to be  -- or to be
>  indistinguishable from -- self-righteous sixteen-year-olds
>  possessing infinite amounts of free time."
>                              -Neil Stephenson, _Cryptonomicon_
>
>