Re: howto preallocate to minimize fragmentation

[Eric Sandeen <sandeen@xxxxxxx>]

Ying-Hung Chen wrote:
> > I guess while you played your video file, which was 2GB. Your video
> > images were not smoothly
> > shown on the player. It gave you an impression that there was a problem
> > on the filesystem.
>
> actually, performance is the problem got my attention, and when i run
> xfs_db and check the fragmentation, its well over 95% on one 200GB
> partition......
run xfs_bmap on a few files, and see how bad it really is.  That number can be 
misleading, sometimes.
but preallocation is an easy way to mitigate any fragmentation problems.

something like this:

int main(int argc, char **argv)
{
        int             fd, err;
        xfs_flock64_t   fl;

        fl.l_whence = 0;
        fl.l_start = 1;
        fl.l_len = (off64_t)(20 * 1024 * 1024); /* 20M */

        if(argc != 2)
                return fprintf(stderr, "Filename expected.\n"), 1;

        fd = open(argv[1], O_RDWR|O_CREAT, 0666);
        if (fd < 0) {
                perror("open: ");
                printf("open of file %s failed\n", argv[1]);
                exit(1);
        }

        err = xfsctl(argv[1], fd, XFS_IOC_RESVSP64, &fl);
        if (err < 0) {
                perror("allocsp: ");
                printf("XFS_IOC_RESVSP64 failed\n");
                close(fd);
                exit(1);
        }

        close(fd);

        return 0;
}

-Eric