Re: [Xen-devel] Starting QEMU as disk backend for dom0

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-06-06 at 15:55 +1000, Steven Haigh wrote:
> Hi guys,
> 
> I'm just picking through the xencommons initscript provided with the Xen
> 4.4 source tarball...
> 
> I notice this section starting at line 119:
>         echo Starting QEMU as disk backend for dom0
>         test -z "$QEMU_XEN" && QEMU_XEN="${LIBEXEC}/qemu-system-i386"
>         $QEMU_XEN -xen-domid 0 -xen-attach -name dom0 -nographic -M
> xenpv -daemonize \
>                 -monitor /dev/null -serial /dev/null -parallel /dev/null \
>                 -pidfile $QEMU_PIDFILE
> 
> Can anyone tell me the purpose of this?

It is used if dom0 needs to be able to access a virtual disk in a
non-trivial form, i.e. qcow or vhd. In this case a vbd is attached to
dom0 which is backed by qdisk in this qemu process. For trivial disk
formats like raw files or block partitions dom0 can just access them
directly.

This comes into play when you are using pygrub, since dom0 needs to be
able to read guest disks. I'm not sure if their are other places. I
suppose you can also do "xl block-attach 0 <spec>" if you like e.g. in
order to mount a qcow image in dom0 for some reason.

Ian.

>  I currently get an error due to
> missing qemu-system-i386 on my system (everything still works?) - but
> for the life in me, I can't figure out what this is meant to achieve :\
> 
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> http://lists.xen.org/xen-devel



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