Raw I/O question

From: Kramer, James P. <james.p.kramer_at_unisys.com>
Date: Tue, 28 Dec 2004 13:00:39 -0600
Message-ID: <19D0D50E9B1D0A40A9F0323DBFA04ACC15B50C@USRV-EXCH4.na.uis.unisys.com>

I now have another question pertaining to I/O. In my previous question, = I was having problems getting Direct I/O to work. With your help, I was = able to get that working. Now I want to compare Raw I/O and Direct I/O = and see if there is any difference.

I bound a disk device to a raw device by typing

raw /dev/raw/raw1 /dev/sde1

This worked. I then changed the code below to use "/dev/raw/raw1" as = the file that gets opened. I needed to have the right privileges to get = past the EACCES error in the open. Now I am receiving an EBUSY (16) = error on the open. I tried changing the flags to O_RDWR | O_NONBLOCK = and that didn't work. Is there another way to do raw I/O? I assumed = that this is the way to do it. Any other gotchas that I forgot about?

Thanks again,

James Kramer

THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY = MATERIAL and is thus for use only by the intended recipient. If you = received this in error, please contact the sender and delete the e-mail = and its attachments from all computers.=20

-----Original Message-----
From: Magni Fabrizio [mailto:Fabrizio.Magni_at_rasnet.it] Sent: Monday, December 13, 2004 9:19 AM
To: Kramer, James P.; oracle-l_at_freelists.org Subject: RE: Direct I/O questions

Hello James,
eventually I was able to make O_DIRECT work even on x86_64 (kernel 2.6.5-7.97-smp and 2.6.9 vanilla).
Unfortunately I haven't got any itanium where to test it.

I had to create a file named xxx in the same directory of the executable with:

dd if=3D/dev/zero of=3Dxxx bs=3D4196 count=3D100

I tested on ext2, ext3, reiserfs.

Regards
Fabrizio

The code (Thanks to Andrea Arcangeli for the revision):

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <stdlib.h>

#define O_DIRECT 040000 /* direct disk access hint */

int main(int argc, char *argv[])
{

     int fd =3D open("xxx", O_RDWR|O_DIRECT);
     int len, pagesize =3D getpagesize();
     char *message;

        printf("Pagesize: %d\n", pagesize);

     posix_memalign((void **)&message, pagesize, pagesize);
     memset(message, 0xff, pagesize);
     printf("%p\n", message);
     if(fd < 0) {
         printf("Unable to open file, errno is %d.\n", errno);
     } else {
         if((len =3D read(fd, message, pagesize)) < 0) {
            perror("read");
          } else {
            printf("%d bytes read from file.\n", len);
            printf("Message: %s", message);
          }

    }
    close(fd);

    return 0;
}

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Received on Tue Dec 28 2004 - 12:59:53 CST