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: [EMAIL PROTECTED]:/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 [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/