RE: Raw I/O question

From: Magni Fabrizio <Fabrizio.Magni_at_rasnet.it>
Date: Tue, 4 Jan 2005 10:36:18 +0100
Message-ID: <BFCF159FFD14F444B205968F32DBD5BB6A5464@MAILEX01.rasgroup.ita.localnet>

Sorry,
I was on vacation. :)

Below you can find the program modified to be tested with raw devices.

It could take long if you have a large device so I suggest you to bind raw1 to a small partition.

Regards
Fabrizio

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <asm/fcntl.h>
#include <errno.h>
#include <string.h>
#define BUFFSIZE 65536
#define BLKSIZE  4096
#define ALIGN    4096

main() {
unsigned char * buff;
int stat1=3D0,stat2=3D0,stat3=3D0;
int fd1=3D0,fd2=3D0;
=20

   buff=3D(char *)(((unsigned long)buff + (BLKSIZE - 1)) & (~(BLKSIZE - 1)));

   if (stat3=3Dposix_memalign(&buff,ALIGN,BUFFSIZE)) {

      fprintf(stderr,"ALIGN ERR:%s\n",strerror(stat3));
      exit(0);

   }

   fd1=3Dopen("/dev/raw/raw1", O_RDONLY);

   if ((lseek(fd1, 0, SEEK_SET)) < 0){

        perror("lseek"); // problem if 2nd arg > 2G
        exit (1);
        }


   while(stat1=3Dread(fd1,buff,BLKSIZE)) {
         if (errno) {
            fprintf(stderr,"%d READ ERR:%s\n",stat1,strerror(errno));
            exit(0);
         }

   }
   close(fd1);
}

> -----Original Message-----
> From: Kramer, James P. [mailto:james.p.kramer_at_unisys.com]=20
> Sent: Tuesday, December 28, 2004 8:01 PM
> To: Magni Fabrizio; oracle-l_at_freelists.org
> Subject: Raw I/O question

>=20
>=20

> I now have another question pertaining to I/O. In my=20
> previous question, I was having problems getting Direct I/O=20
> to work. With your help, I was able to get that working. =20
> Now I want to compare Raw I/O and Direct I/O and see if there=20
> is any difference.

>=20

> I bound a disk device to a raw device by typing

>=20

> raw /dev/raw/raw1 /dev/sde1

>=20

> This worked. I then changed the code below to use=20
> "/dev/raw/raw1" as the file that gets opened. I needed to=20
> have the right privileges to get past the EACCES error in the=20
> open. Now I am receiving an EBUSY (16) error on the open. I=20
> tried changing the flags to O_RDWR | O_NONBLOCK and that=20
> didn't work. Is there another way to do raw I/O? I assumed=20
> that this is the way to do it. Any other gotchas that I forgot about?

>=20

> Thanks again,

>=20

> James Kramer

>=20

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

>=20
>=20
>=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

>=20
>=20

> 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.

>=20

> I had to create a file named xxx in the same directory of the=20
> executable
> with:

>=20

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

>=20

> I tested on ext2, ext3, reiserfs.

>=20

> Regards
> Fabrizio

>=20

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

>=20


> #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>

>=20


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

>=20
>=20

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

>=20


>      int fd =3D open("xxx", O_RDWR|O_DIRECT);

>      int len, pagesize =3D getpagesize();

>      char *message;

>=20


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

>=20


>      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);

>=20


>     return 0;

> }

>=20
>=20

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Received on Tue Jan 04 2005 - 03:31:34 CST