Re: [Xen-users] domUs with iSCSI disks... connect in dom0 or domU?

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

xenusers wrote:

> I don't know if this is something that is a simple matter of opinion 
> or if there are strong reasons to take one route or the other.
There are good performance reasons ...

> Currently I have a few domUs running that have large-ish volumes on 
> the iSCSI SAN, and to present the volumes I'm connecting the dom0s 
> to the storage, then in the domU config I give it the iSCSI volumes 
> as 'phy:/dev/disk/by-path/ip-10.10.10.5-iqn....:volume1' and so on.
> I'm wondering if instead it's considered better to simply have the 
> domU make those iSCSI connections, by giving it a second interface 
> that connects to a bridge with access to the iSCSI VLAN?  I set one 
> up that way instead, and it works as expected though it seems to 
> perform less well in some admittedly simple tests using dd with 
> 'oflag=direct' set.
It's been a topic of discussion here on and off in the past. 
Basically, as you found, attaching the iSCSI volume in the DomU 
results in a big performance hit, although IMO it is a much 
preferable setup as it isolates the guest requirements from the host 
and simplifies moving a guest.
AIUI, the networking in Xen is single threaded, and so your iSCSI 
traffic competes for this single thread with all the other traffic. I 
don't know about disks, but I strongly suspect support for them is 
multi-threaded.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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