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
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