Re: Slow disks.

[Rogier Wolff <R.E.Wolff@BitWizard.nl>]

On Wed, Dec 22, 2010 at 11:27:20AM -0500, Jeff Moyer wrote:
> Rogier Wolff <R.E.Wolff@BitWizard.nl> writes:
> 
> > Unquoted text below is from either me or from my friend. 
> >
> >
> > Someone suggested we try an older kernel as if kernel 2.6.32 would not
> > have this problem. We do NOT think it suddenly started with a certain
> > kernel version. I was just hoping to have you kernel-guys help with
> > prodding the kernel into revealing which component was screwing things
> > up....
> [...]
> > ata3.00: ATA-8: WDC WD10EARS-00Y5B1, 80.00A80, max UDMA/133
> 
> This is an "Advanced format" drive, which, in this case, means it
> internally has a 4KB sector size and exports a 512byte logical sector
> size.  If your partitions are misaligned, this can cause performance
> problems.

This would mean that for a misalgned write, the drive would have to
read-modify-write every super-sector. 

In my performance calculations, 10ms average seek (should be around
7), 4ms average rotational latency for a total of 14ms. This would
degrade for read-modify-write to 10+4+8 = 22ms. Still 10 times better
than what we observe: service times on the order of 200-300ms. 

 > md1 : active raid5 sda2[0] sdd2[3](S) sdb2[1] sdc2[4]
> >       39067648 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3]
> > [UUU]
> 
> A 512KB raid5 chunk with 4KB I/Os?  That is a recipe for inefficiency.
> Again, blktrace data would be helpful.

Where did you get the 4kb IOs from? You mean from the iostat -x
output?  The system/filesystem decided to do those small IOs. With the
throughput we're getting on the filesystem, it better not try to write
larger chuncks...

I have benchmarked my own "high bandwidth" raid arrays. I benchmarked
them with 128k, 256, 512 and 1024k blocksize. I got the best
throughput (for my benchmark: dd if=/dev/md0 of=/dev/null bs=1024k)
with 512k blocksize. (and yes that IS a valid benchmark for my
usage of the array.)

	Roger. 

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