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Subject: Re: Using DMA in read/write, setting block size for I/O
Date: Tue, 4 Oct 2005 11:34:52 -0400
Message-ID: <Pine.LNX.4.61.0510041127550.28393@chaos.analogic.com>
Thread-Topic: Using DMA in read/write, setting block size for I/O
Thread-Index: AcXI+SpU3yXXIPxyQwieG3E+aiqK9Q==
From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Karthik Sarangan" <karthiks@cdac.in>
Cc: <linux-kernel@vger.kernel.org>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
Sender: linux-kernel-owner@vger.kernel.org
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On Tue, 4 Oct 2005, Karthik Sarangan wrote:

> In my application, I have the following code.
>
> int main(void)
> {
>    char *pcBuffer;
>
>    posix_memalign((void **) pcBuffer, 512, 262144);
>    int ifd = open("/dev/sdb", O_DIRECT | O_RDWR);
>    long lLen;
>
>    lLen = read(ifd, pcBuffer, 262144);
>
>    close(ifd);
>    return 0;
> }
>
> Will the underlying block device read a single 256KB block from the hdd
> into pcBuffer

No. It might read 512 bytes! You can't assume that a read will
always return the value requested. That's a bug. You need code
that will make as many reads as necessary to satisfy your
request.

> or will it read 256KB as a set of smaller blocks?
>

You (your code) may have to read multiple times. The
kernel code may even read ahead.

> Since the buffer is memory aligned will it enable DMA?
>
> scsi disk driver is adaptec aic79xx.o

This Adaptec SCSI driver always uses DMA.

> distro is RedHat Enterprise Linux WS 4 (kernel-2.6.9-11)
> -


Cheers,
Dick Johnson
Penguin : Linux version 2.6.13 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.

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