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RE: AW: [Xen-users] Best way to use iSCSI in domU

  • To: "Simon Hobson" <linux@xxxxxxxxxxxxxxxx>, <Xen-users@xxxxxxxxxxxxxxxxxxx>
  • From: Jeff Sturm <jeff.sturm@xxxxxxxxxx>
  • Date: Thu, 18 Jun 2009 13:43:45 -0400
  • Cc:
  • Delivery-date: Thu, 18 Jun 2009 10:47:24 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: AcnwGJmt1DXij9W0QZCgpG1JOS45CgAIirUA
  • Thread-topic: AW: [Xen-users] Best way to use iSCSI in domU

We had a similar experience, however with AoE rather than iSCSI.  The
xennet drivers were fast when benchmarked on a single DomU.  We hadn't
anticipated the slowdown when many DomU's were hitting xennet for block
storage, cluster multicast and plain HTTP traffic all at once.

Exporting block devices from DomU instead was much more successful.  The
performance bottleneck seemed to vanish, we could tune the network as we
see fit (e.g. multipath / jumbo frames, good for storage), and the phy:
backend allows for asynchronous writes (although the latter prevented us
from doing this on shared GFS filesystems).

This was on CentOS 5.2, in case anyone is wondering.


-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Simon Hobson
Sent: Thursday, June 18, 2009 9:28 AM
To: Xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: AW: [Xen-users] Best way to use iSCSI in domU

Rustedt, Florian wrote:
>Why are you thinking about implementing another "hop" via dom0? What 
>is the advantage you asume in this case?

Performance !
I'm not alone in finding iSCSI performance was "rather poor" 
connecting from the DomU. I reluctantly configured iSCSI in the Dom0 
and passed the resulting SCSI device as a block device to the DomU 
and performance was "quite acceptable". From bits I've picked up here 
and there, I believe it's something to do with the network stack in 
XEN/Dom0 being single threaded and killing performance - though I 
don't know why it should affect iSCSI and not other networking.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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