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Re: Setting up vnd devices for disks



Louis Guillaume <louis%zabrico.com@localhost> writes:

> Ok - I get the overall way the block devices or files are mapped into
> the guest machines. But I'm still confused here:
>
> I can create a regular file and make it available as a disk using
>
>   disk = [ 'file:/srv/xen/host-disk0,0x1,w' ]
>
> ... as you described. But why do we need the vnd device? Just
> accessing the file itself does the trick for domU. Is some
> vnconfigging going on automatically that I don't know about? I can

Yes, the startup script finds a vnd and vnconfigs it and points the domU
at the vnd.  This is far easier than doing it explicitly.  see the logs.

> also do this...
>
>   disk = [ 'phy:/dev/vnd0d,0x05,w' ]
>
> ... and get effectively the same results, except that I'm
> *referencing* the vnd device in the config. Essentially that means ALL
> my vnd devices must be configured BEFORE I start up the guests.

yes, this is why the script is nice.

> What would be the benefit of the latter situation? If I wanted to
> configure any of my files as vnd devices I can just vnconfig a vnd
> device on those files and voila! I see everything; partition map;
> disklabel - everything.
>
> I still don't see any reason for vnd here.

Because it won't work without it, because (apparently) the dom0 kernel
has to be handed a block device.

> Ok - I get this. We can vnconfig in dom0 to access the data in our
> guest system. That is a perfectly valid use here. But it hasn't
> anything to do with xen, or the xen configuration/booting environment.

true.

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