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Re: [Xen-users] Speed of Xen Network Bridge Interface (10/100/1000?)

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Subject: Re: [Xen-users] Speed of Xen Network Bridge Interface (10/100/1000?)
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 27 Nov 2007 22:14:48 +0000
Cc: Emre Erenoglu <erenoglu@xxxxxxxxx>, Javier Guerra <javier@xxxxxxxxxxx>
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> > More specifically, on xenbr0, given that the traffic only occurs between
> > my Dom0 host and PV DomU guest, am I limited to 10 Mbps, 100 Mbps or 1000
> > Mbps? Does it depend on something such as ethernet card capability (even
> > though the packets don't go out of the card and stay inside Dom0)?
> >
> > I plan to use iScsi or Ata-over-Ethernet, that's why I'm asking this
> > question,
> AFAIK, there's no hard speed limit.  on HVM guests, the eth card
> appears as a 10Mbit model, but usually performs much faster than that.

HVM incurs more overhead, so for HVM virtual ethernet it'll be slower than if 
you use the PV virtual ethernet.

The overhead for HVM is due to the inefficiencies of emulating real hardware 
under Xen, and should not (as Javier points out) be limited by the fact that 
the device pretends to be 10Mbps.


> OTOH, any block device protocol with the initiators on DomU is
> certainly doable, but you get huge overheads.  be sure to dedicate at
> least one physical CPU core to Dom0, so that you don't get two
> context-switchs (with associated blocking) for each packet.
> IMHO, you always want to use highest-level, biggest-block abstractions
> on DomU's, with any low-level, small-block implementations terminated
> at Dom0.  in your case, the iSCSI initiator or AoE module would be on
> Dom0, and DomU's would see only block devices, without any special
> drivers.
> why would you want to run AoE/iSCSI over the emulated LAN? is it to
> make it easier to live-resize the block devices?  there should be a
> better way...

Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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