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

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

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.

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