This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


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

To: Steffen Heil <lists@xxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] Very slow drives, I/O - DMA issues?
From: Tim Post <tim.post@xxxxxxxxxxxxxxx>
Date: Thu, 22 Feb 2007 15:34:51 +0800
Cc: 'Lonny Selinger' <lonny@xxxxxxxxxxxxxxxxxxxxxxxx>, burke@xxxxxxxxxxxxxxxxx, xen-users@xxxxxxxxxxxxxxxxxxx
Delivery-date: Wed, 21 Feb 2007 23:34:49 -0800
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
In-reply-to: <00ac01c755f7$80d08220$0b4613ac@shs1>
List-help: <mailto:xen-users-request@lists.xensource.com?subject=help>
List-id: Xen user discussion <xen-users.lists.xensource.com>
List-post: <mailto:xen-users@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=unsubscribe>
Organization: Net Kinetics
References: <39120.> <59303.> <37633.> <6d06ce20702201754q256119bdnb4124106572136e7@xxxxxxxxxxxxxx> <35464.> <19542.> <19559.> <6d06ce20702210753k27fdec7v830820d5cb144f12@xxxxxxxxxxxxxx> <27188.> <36062.> <28571.> <00ac01c755f7$80d08220$0b4613ac@shs1>
Reply-to: tim.post@xxxxxxxxxxxxxxx
Sender: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
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 :)


Xen-users mailing list