Re: write is faster whan seek?

[Jens Axboe <jens.axboe@oracle.com>]

On Wed, Jun 11 2008, Dmitri Monakhov wrote:
> I've found what any non continious sequence violation  result in significant
> pefrormance drawback. I've two types of requests:
> 1)Ideally sequential  writes:
>    for(i=0;i<num;i++) {
>        write(fd, chunk, page_size*32);
>    } 
>    fsync(fd);
> 
> 2) Sequential writes with dgap for each 32'th page
>    for(i=0;i<num;i++) {
>        write(fd, chunk, page_size*31);
>        lseek(fd, page_size, SEEK_CUR);
>    }
>    fsync(fd);
> 
> I've found what second IO pattern is about twice times slower whan the
> first one regardless to ioscheduler or HW disk. It is not clear to me
> why this happen. Is it linux speciffic or general hardware behaviour
> speciffic.  I've naively expected what disk hardware cat merge several
> 31-paged requests in to continious one by filling holes by some sort
> of dummy activity.

Performance should be about the same. The first is always going to be a
little faster, on some hardware probably quite a bit. Are you using
write back caching on the drive? I ran a quick test here, and the second
test is about ~5% slower on this drive.

-- 
Jens Axboe