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Re: [Xen-users] Does I/O from Dom0 skip the PV backend drivers ?

To: Xen List <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Does I/O from Dom0 skip the PV backend drivers ?
From: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Date: Thu, 30 Apr 2009 10:29:14 +0700
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On Thu, Apr 30, 2009 at 3:45 AM, Joe Armstrong <Joe.Armstrong@xxxxxxxxx> wrote:
> So the question boils down to – When Dom0 performs I/O does it go directly
> to the physical device driver

Should be. There is an exception though.

> or does it also route through the PV backend
> drivers ?

The exception is that if you deliberately use netfront/blkfront on
dom0. Examples :
- the old (3.0, also in RHEL) Xen bridged networking setup where
dom0's eth0 is renamed to peth0, and veth0 is renamed to eth0. In this
case network traffic goes through netfront/netback drivers.
- mapping a storage to dom0 via "xm block-attach". For example, running
xm block-attach 0 tap:aio:/var/lib/xen/root.img xvda
would create a block device called xvda on dom0, which goes through
blkfront/blkback drivers. Useful if you need to mount domU's storage
on dom0.

> Also, my assumption is that the xen hypervisor has no role in any I/O
> operations so it being present or not should have almost no impact on I/O
> performance.

In current implementation, correct. This post from Mark WIlliamson
might be of interest to you
Note that the hypervisor still have some impact on CPU performance, so
if your benchmark is cpu-bound you might get different result from
native Linux compared to xenified Linux.



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