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From: "colin" <colin@realtek.com.tw>
To: <linux-kernel@vger.kernel.org>
Subject: Re: A problem about DIRECT IO on ext3
Date: Mon, 29 Aug 2005 21:21:11 +0800
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Hi all,
Sorry, ignore this mail.
I found that I didn't align the block size when doing direct io...  :-(

Regards,
Colin


----- Original Message ----- 
From: "colin" <colin@realtek.com.tw>
To: <linux-kernel@vger.kernel.org>
Sent: Monday, August 29, 2005 8:15 PM
Subject: A problem about DIRECT IO on ext3


>
> 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);
> }
>
>
>
>
>