Messages in this thread | | | Date | Mon, 29 Aug 2005 15:29:47 +0200 | From | Erik Mouw <> | Subject | Re: A problem about DIRECT IO on ext3 |
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On Mon, Aug 29, 2005 at 08:15:43PM +0800, colin wrote: > 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.
No, it's right, but it's not what you expected.
> It should be 3 times the size of directio because there are 2 write > operations and one lseek to the file end.
I suggest to strace() your program to see what happens.
> 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.
There are four prerequisites for direct IO: - the file needs to be opened with O_DIRECT - the buffer needs to be page aligned (hint: use getpagesize() instead of assuming that a page is 4k - reads and writes need to happen *in* multiples of the soft block size - reads and writes need to happen *at* multiples of the soft block size
> Is there any problem of lseek when doing direct io on ext3? > My platform is 2.6.11.
There is no problem.
> Regards, > Colin > > #define _GNU_SOURCE > > #include <stdio.h> > #include <fcntl.h> > #include <sys/stat.h> > #include <sys/types.h> > #include <stdlib.h>
Compile your program with -Wall, you're missing quite some include files over here.
> 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));
Make that lseek(fd2, 4 * 4096, SEEK_SET);
> ptr1 = malloc(4096 + 4096-1); > ptr2 = (void*)((int)ptr1 - (int)ptr1 % 4096 + 4096);
Use memalign() or posix_memalign().
> do { > count = read(fd1, ptr2, 4096); > if(!count) > break;
And what happens when count < 0 ?
> write(fd2, ptr2, 4096); > write(fd2, ptr2, 4096);
Check return values.
> } while(count > 0); > > free(ptr1); > close(fd1); > close(fd2);
return 0;
> }
With the changes, the result is:
erik@arthur:/tmp > ls -l directio aaa -rwxr-xr-x 1 erik erik 49152 2005-08-29 15:26 aaa* -rwxr-xr-x 1 erik erik 12628 2005-08-29 15:26 directio*
Erik
-- +-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 -- | Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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