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Re: 'Wide Area' RAC nodes

From: K Gopalakrishnan <>
Date: Tue, 26 Jun 2007 15:50:50 -0000
Message-ID: <>


I have implemented RAC on Wide Area Network Clusters at few customer sites (which is known as 'extended RAC' inside oracle!), and it requires careful planning and execution. You have to have shared storage in both the nodes and sometimes a third node for voting disk (to correctly detect the 'split brain' during network failures). You can either use a host mirroring or array mirroring depending on the hardware/OS/ASM choice.

> Is anybody aware of these products being used to create 'wide area'
> RAC clusters? If so I'd be interested to hear opinions.

It can also be considered as a limited DR solution, however there may be a performance penalty if you extend the clusters over 100 miles (and eventually that is the largest distance I have known on 'extended RAC'). This is due to the 'speed of light factor' as explained in the 'Oracle RAC Handbook'.

**QUOTE** 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 QUOTE** Received on Tue Jun 26 2007 - 10:50:50 CDT

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