[CentOS] Extremely slow disk transfers with SATA disks

[vanja at pobox.com]

On Thu, Mar 23, 2006 at 04:54:44PM +0100, Peter Kjellström wrote:
> On Thursday 23 March 2006 15:37, Vanja Hrustic wrote:
> > I have installed CentOS 4.2, recently, on new computer.
> >
> > Everything seemed to work fine.
> >
> > However, we've found out that disk transfers are incredibly slow, and I
> > just can't figure out what to do in order to fix it.
> >
> > Computer is now running CentOS 4.3 (updated it 1 hour ago), and same
> > thing is still present. I hoped kernel upgrade might fix it, but it
> > didn't.
> 
> Very interesting, you do have working sata drivers (your device is /dev/sd*) 
> and your hdparm -t gives expected number and yet you see really bad 
> performance... wow.
> 
> Have you tried to figure out if it's reading or writing or both that suck? try 
> this:
> 
> time dd if=/dev/zero of=testfile bs=1M count=1000
> 
> time dd if=testfile of=/dev/null bs=1M
> 
> If you have 1 gig ram or more adjust count...

As usual, I get to test more things only after I make a post to the mailing list :)

What I found out is that 1 disk is creating problems (out of those 2), and only when reading.

So, that 200GB Maxtor has no problems writing (300MB file gets written in few seconds, when it's read from another 80GB SATA 
disk), but when I read something from that disk - I get 2MB/sec. I have no trouble reading from 80GB SATA disk, so I guess 
it will be cable/hardware related.

Also, I said it works ok in Windows, but Windows partition is located on 80GB drive. 200GB drive only holds Linux 
partitions.

I have no idea why hdparm reports 50MB/sec rates on 200GB disk, but I'll just assume that hdparm doesn't use "real life" way 
of testing speed :)

Thanks for feedback. I will post again when/if I fix this, and figure out if it was indeed a hardware problem.

Take care.

Vanja