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

[Ross Walker <rswwalker@xxxxxxxxx>]

On Nov 13, 2008, at 5:58 PM, Simon Hobson <linux@xxxxxxxxxxxxxxxx>  
wrote:
> John Madden wrote:
>
> > I'd try moving it to dom0 so only the block i/o component of the  
> > picture goes
> > over the hypervisor.  You're doing a lot of double-work by doing  
> > both block
> > i/o and the networking needed for iscsi when you do the initiator  
> > in domU.
> > Configuration will be easier, too.
> Not sure what you are saying about the double work - with the DomU  
> doing the iSCSI, all the Dom0 has to do is pass the network packets  
> through. Moving the iSCSI initiator to the Dom0 means that the Dom0  
> has to do the iSCSI stuff and then route the virtual block device to  
> the DomU.
> I've deliberately not put the iSCSI initiator in Dom0 as I want to  
> run the absolute minimum in Dom0. Also, putting the iSCSI initiator  
> in Dom0 makes it harder to move a VM to another host - having it in  
> the DomU means that the VM can be moved without any config changes.
Try running netperf between dom0 and domU to see how well the  
networking is working, then between domU and the iSCSI target (or  
another external host).
There have been weird problems with vlans too, dropping ids on  
forwards depending on the source card and how it pads and such. Google  
for Xen network performance issue/problem.
-Ross


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