RE: [Xen-users] Very slow drives, I/O - DMA issues?

[Tim Post <tim.post@xxxxxxxxxxxxxxx>]

On Wed, 2007-02-21 at 21:33 +0100, Steffen Heil wrote:
> Hello.
> 
> I saw your post:
> > /dev/hda:
> >  Timing cached reads:   1380 MB in  2.00 seconds = 689.47 MB/sec
> >  Timing buffered disk reads:  142 MB in  3.02 seconds =  47.02 MB/sec
> > 
> > /dev/hdb:
> >  Timing cached reads:   1452 MB in  2.00 seconds = 725.20 MB/sec
> >  Timing buffered disk reads:  138 MB in  3.02 seconds =  45.68 MB/sec
> 
> I wondered about hdparm -tT, which I didn't know.
> So I tries it myself:

I tried that as well as a quick and dirty benchmark to see how closely
it matched on dom-0 to a dom-u. 

I noticed that cached reads were slightly slower on the guest (by about
8 - 12%), but buffered reads were actually faster than dom-0 (by a
small, but noticeable 3 - 5%).

Never could quite explain it either, was just content that it was close
and similar to readings on standard stock Linux systems. 

I'm kind of curious about this too. I mostly understand it, but not well
enough to explain it to someone who does not, which means I don't fully
understand it but would like to :)

Best,
--Tim



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