How to increase web server uptime with DNS failover

Posted by Guy Macon on October 26, 2008, 1:11 pm

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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?t=524788
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?IDTXT=36

--
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/>

Posted by Guy Macon on October 26, 2008, 4:16 pm

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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?t=524788

>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?IDTXT=36

More References:
http://www.simplefailover.com/outbox/dns-caching.pdf
http://ausweb.scu.edu.au/aw03/papers/wingenroth/paper.html
http://support.easydns.com/Failoverfaq.php
http://edgedirector.com/htm/note.htm

--
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/>

Posted by viza on October 27, 2008, 2:51 pm

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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.

Posted by Guy Macon on October 27, 2008, 7:09 pm

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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_

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

Posted by Jerry Stuckle on October 27, 2008, 8:21 pm

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Guy Macon 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

> 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.

But he's right - both systems share the same internet. Completely
different than your cell phone and laser printer. This includes, among
other things, the DNS system, which isn't necessarily independent of
other parts of the internet.

They are a common point of failure.

> 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_

--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

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