Re: DBWR performance

From: Mladen Gogala <>
Date: Fri, 18 May 2012 12:17:49 +0000 (UTC)
Message-ID: <>

On Wed, 16 May 2012 20:53:48 +1000, Noons wrote:

> Didn't Oracle at some stage do something fancy with their own flavour of
> Linux?

Yes. They started touting RAC. RAC cannot solve the IO bottleneck but can make much more money for Oracle.

> Yeah. And the backplane on the iPower series also has some
> accountability in all that. I am continually surprised at how much
> processing throughput our DW is pulling.

Of course it does. True minicomputers and mainframes, the categories that have been merged into one by the market pressure of cheap Linux systems, do IO in a different way than PC based machines. The differences are IO channels which are much cheaper from the processing perspective, but require a proprietary hardware ("channel controller") which takes care of the IO. On the true minicomputer, one puts the IORB (IO request block) into the special location, monitored by the channel processor. The channel processor picks the request up, executes it and sends a single interrupt to the CPU saying "I'm done, take a look". The IORB is marked as complete. That's it. PC hardware is very chatty: CPU sends a message to the IO controller: are you ready to receive? Controller responds with: no, I am busy or yes, send the request. Then CPU sends the request, after which the controller completes the request, puts it in memory and sends a message that it is done. Each message is called "an interrupt" and has certain execution characteristics, like executing in kernel context and preventing other interrupts from getting delivered. Much more time is wasted than is the case with channel architecture. Unfortunately, PCI-X and SATA are interrupt based and all the vendors who produce equipment want it to remain so, for compatibility reasons. That is why the I2O attempt of addressing this performance shortcoming has failed. However, if you stack up enough of Dell boxes together, you will get the same capacity as with a single iPower box, especially if you use Exadata. I am not sure that the price will remain low, however. Hardware is now dirt cheap and for a price of a single Oracle license, I can buy 10 very good PC's. Having one powerful iPower machine with 8GB or even 16GB FC/AL interfaces and a decent general purpose SAN for DW is probably much more cost effective than having 4 way RAC, with Exadata. Performance should be comparable, if not better.

Received on Fri May 18 2012 - 07:17:49 CDT

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