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

  • To: "Mark Williamson" <mark.williamson@xxxxxxxxxxxx>
  • From: "Emre Erenoglu" <erenoglu@xxxxxxxxx>
  • Date: Thu, 29 Nov 2007 02:03:06 +0100
  • Cc: xen-users@xxxxxxxxxxxxxxxxxxx, Javier Guerra <javier@xxxxxxxxxxx>
  • Delivery-date: Wed, 28 Nov 2007 17:03:58 -0800
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Hi, thanks to everybody who commented on my question, now it's clear for me.

Let me explain a little deeper why I asked this question (AoE issue). While surfing on the net and looking for solutions how to run PV drivers inside a HVM DomU linux guest (like the PV drivers for Windows), I came accross this website:

This is explaining in some detail how to build and use PV drivers for storage and networking. Everything's fine.

However, reading through, even though you can use this PV VBD driver for other filesystems, you can't use it for the  "root" device .  Show begins here :) 

As far as I understood from the text, the PV networking driver can be enabled at boot time, so I had this crazy idea of using AtaOverEthernet to boot up the system through the PV network card. I don't know if it's a good idea anyways, but I wanted to try.

Of course the issue here would be, if the PV driver based network card can sustain a disk throughput similar to the Dom0 throughput, or would I be somewhat limited to the speed of the card or driver itself... (appearantly not).

One last thing, I asked to Jan (owner of text) about it, he commented that it's not possible to use the PV VBD on "boot" device, but not on other file systems, so if my / "root" filesystem resides on a different VBD device, it may work. I will also try this. I will just put ioemu device for the /boot partition, the rest will be PV VBD.

What do you think?



On Nov 27, 2007 11:14 PM, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > 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!

Emre Erenoglu
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