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

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-' 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

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author Gladys Hobson. Novels - poetry - short stories - ideal as
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