WARNING - OLD ARCHIVES

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/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-users

[Xen-users] Performance of network block devices (iSCSI)

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] Performance of network block devices (iSCSI)
From: Simon Hobson <linux@xxxxxxxxxxxxxxxx>
Date: Thu, 13 Nov 2008 19:32:38 +0000
Delivery-date: Wed, 17 Dec 2008 08:23:32 -0800
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
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/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=unsubscribe>
Sender: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
I have a 'backup' server to which I have a number of machines dumping their filesystems using rdiff-backup. The backup server is storing this data on a volume mounted off an iSCSI store (Dell/EMC AX100i). I've found the performance to be 'very poor' and asked on the rdiff-backup list, a response I got was :

I found that the network and I/O scheduler in Xen was a single pipeline and contention was terrible. We got terrible performance when we used network block devices with Xen, as the VMs would just sit in waitI/O all the time when accessing the network block devices (we tried AoE, NBD, iSCSI).
...
We ended up moving to OpenVZ and haven't looked back.

I've done a test after copying the store to a local disk (xvda) which is another volume in the LVM setup of the Xen host - it's notable that copying the backup off the iSCSI volume ran at only about 1/2G/hr. The difference is quite dramatic, a backup from one client takes 36s to a local disk, but 9 1/2 minutes to the iSCSI box - that's a 15 fold difference.

While copying to or from the iSCSI volume the backup server sits at 100% (occasionally 99%) wait-io, while backing up to the virtual disk it shows the normal levels of processor activity I would expect (with minimal wait-io).

Systems are Debian Lenny, running on a Dell 2650 with hardware raid (PERC) and plenty of RAM.

Is there something I've missed ? Is there anything I can do ?

--
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.

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users

<Prev in Thread] Current Thread [Next in Thread>
  • [Xen-users] Performance of network block devices (iSCSI), Simon Hobson <=