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

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Rustedt, Florian wrote:

> That depends on your scenario. If you've got more than one 
> dom0(Xen-Servers), exspecially for redundancy, then it's not a good 
> idea to make dom0 the iscsi-host.
> Of course, you can connect to any iscsi-target directly from the 
> domUs, as you can connect the dom0 vias iscsi and share the 
> ressource via NFS p.e. then.
> Me personally, i would connect it directly to the domUs, as this is 
> straightforward.
Much what I thought - attach directly from the DomU and then the 
connection is portable wherever you host that DomU. It didn't work 
well for me (see below).
> 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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