Mailinglist Archive: opensuse (5130 mails)

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Re: [SLE] Re: DMA, ATA standards, etc... -- WAS: Advantages of Dual Core
  • From: "Carlos E. R." <robin.listas@xxxxxxxxxxxxxx>
  • Date: Mon, 29 May 2006 20:41:08 +0200 (CEST)
  • Message-id: <Pine.LNX.4.61.0605292038340.7523@xxxxxxxxxxxxxxxx>
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The Monday 2006-05-29 at 11:22 -0400, Bryan J. Smith wrote:

> On Sun, 2006-05-28 at 20:04 +0200, Carlos E. R. wrote:
> > Why is that? I'm curious. Can't it use dma, even if it uses smaller
> > writes? Just guessing.
>
> Because all writes are Programmed I/O (PIO) through the CPU.
>
> XOR might be simple, but attempting to deal with the traditional
> LOAD/EXEC/STOR through the CPU -- even using streamed XOR operations --
> at hundreds of MBps is still damn slow through.
>
> CPUs are not I/O processors (IOP). And such IOP operations take away
> from other operations. So unless your system is a dedicated storage
> device, and not servicing anything else (which is fine), you're taking
> away from your other operations.
>
> And, again, it's virtually _impossible_ to measure these metrics in the
> current Linux kernel. All you can find out is I/O wait and servicing
> for processes, not the I/O latency and throughput itself.



That I understand. But my question was in the line of why raid-5 uses, or
has to use, PIO instead of DMA. What is so different for raid-5. I can
guess, but I don't know.

- --
Cheers,
Carlos Robinson

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