A problem about DIRECT IO on ext3

["colin" <colin@realtek.com.tw>]

Hi all,
I wrote a simple program to test direct io, and found that there are some
strange behaviors of it on "ext3".
My simple program is below. Assume that the executable file name is
"directio". If I do the following:
    1. cp directio aaa
    2. ./directio directio aaa

The size of aaa is about the same with directio. This is wrong.
It should be 3 times the size of directio because there are 2 write
operations and one lseek to the file end.

If the second file is not opened with "O_DIRECT", the result is correct.

What's the problem of direct io? I found that if I remove the instruction of
lseek, the result is correct.
Is there any problem of lseek when doing direct io on ext3?
My platform is 2.6.11.

Regards,
Colin








#define _GNU_SOURCE

#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <stdlib.h>


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

int fd1, fd2, count;
char *ptr1, *ptr2;

if(argc == 3) {
  fd1 = open(argv[1], O_RDONLY | O_DIRECT, S_IRWXU);
  fd2 = open(argv[2], O_RDWR | O_CREAT | O_DIRECT);
} else {
  printf("Error syntax\n");
  exit(1);
}
printf("%d\n", lseek(fd2, 0, SEEK_END));

ptr1 = malloc(4096 + 4096-1);
ptr2 = (void*)((int)ptr1 - (int)ptr1 % 4096 + 4096);

do {
  count = read(fd1, ptr2, 4096);
  if(!count)
    break;
  write(fd2, ptr2, 4096);
  write(fd2, ptr2, 4096);
} while(count > 0);

free(ptr1);
close(fd1);
close(fd2);
}





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