# Re: RAC Geographical Architecture

• From: "K Gopalakrishnan" <kaygopal@xxxxxxxxx>
• To: "Michael Fontana" <MFontana@xxxxxxxxx>
• Date: Mon, 16 Apr 2007 16:07:17 -0700

```Mike,

It does not imply 100km limit, the reason is differnt and we call as
'speed of light factor'.  Quoting from my 'Oracle 10g RAC Handbook'
Chapter 15,

***BEGIN***
In vacuum the light travels at 186, 282 miles per second. To make the
math simpler, we round that off to 200,000 miles per second or 200
miles per millisecond. We require confirmation of any message, so we
must use round-trip distances. Therefore, light can travel up to 100
miles away and back in 1 millisecond. But the actual transit time is
longer, in part because the speed of light is slower by about 30% in
an optical fiber and straight lines rarely occur in global
communications situations, but also because delays are created when
the signal passes through an electronic switch or signal regenerator.
While considering all the factors like the inevitable switch delays,
the conservative rule of thumb is that it takes 1 millisecond for
every 50-mile round trip. Therefore, 500 miles adds 10 milliseconds to
the latency of a disk access. Given normal disk access latency of
10-15 milliseconds, this merely doubles the latency, however the
shadowing software layer can cope with that. But if the latency is
more than that, the software might think the disk at the other end has
gone off-line and will incorrectly break the shadow set. So the speed
of light becomes the limiting factor because a latency of 1
millisecond (ms) is introduced for every 50miles of distance between
the data centers, even with the use of dark fiber.

***END***

On 4/16/07, Michael Fontana <MFontana@xxxxxxxxx> wrote:
```
```Someone else wrote back with a link to a good whitepaper from Oracle
published in 2006.

It seemed to imply a 100km limit, between the servers, in geographical
terms.

In addition to the added cost for the DWDM (hundreds of thousands\$\$\$\$),
a site would also need at least four mirrored (remote) copies of disk,
with clustering technology (which ain't cheap).

You seem to imply this has been do-able since 2004?  And that there are
least 10 such sites running it?  Amazing.
```
```
The number is quite higher than 10. To my knowledge it is around 50+
and 10 of them are direclty known to be .

-Gopal

```
```I would imagine 11g may have even more capability.  While I don't
normally like to discuss or even consider vaporware, this time my
question is based upon putting together a 3-5 year plan for my
executives.  Any information on future Oracle direction would be
```
```
I am neither qualified nor allowed to talk more on this !!! But
extended RAC is increasingly common now a days..

```
```-----Original Message-----
From: K Gopalakrishnan [mailto:kaygopal@xxxxxxxxx]
Sent: Monday, April 16, 2007 4:27 PM
To: Michael Fontana
Cc: oracle-l@xxxxxxxxxxxxx
Subject: Re: RAC Geographical Architecture

Mike,

Your instructor is right. There is no limitation from RAC side. There
are various technologies support this and the most common is DWDM (dark
fiber) and there are lot of customers (I have personally
participated/known more than 10+ of such) successfully deployed this
architecture and running without any problems.

In case if you have my RAC book, I have a dedicated chapter for
"Extended RAC". Have a look in near by bookstore

-Gopal

BTW 11g RAC/ASM has some additional features for extended RAC (like
preferred read for asm failure groups), however it would be too
premature to discuss that now !
```
```
--
Best Regards,
K Gopalakrishnan
Co-Author: Oracle Wait Interface, Oracle Press 2004
http://www.amazon.com/exec/obidos/tg/detail/-/007222729X/

Author: Oracle Database 10g RAC Handbook, Oracle Press 2006
http://www.amazon.com/gp/product/007146509X/
--
http://www.freelists.org/webpage/oracle-l

```