Re: write is faster whan seek?

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

On Wed, Jun 11 2008, Dmitri Monakhov wrote:
> Jens Axboe <jens.axboe@oracle.com> writes:
> 
> > 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.
> Hmmm... it is definitly not happen in may case.  
> I've tested following sata drive w and w/o write cache
> AHCI
> ata7.00: ATA-7: ST3250410AS, 3.AAC, max UDMA/133
> ata7.00: 488397168 sectors, multi 0: LBA48 NCQ (depth 31/32)
> In all cases it is about two times slower.
> 
> # time   /tmp/wr_test /dev/sda3 32 0 800
> real    0m1.183s
> user    0m0.002s
> sys     0m0.079s
> 
> # time   /tmp/wr_test /dev/sda3 31 1 800
> real    0m3.240s
> user    0m0.000s
> sys     0m0.078s

Ahh, direct to device. Try making a 4kb fs on sda3, mount it, umount
it, then rerun the test.

-- 
Jens Axboe