Subject: Re: ok to bump vnd to 32 in XEN0 config?
To: Juan RP <>
From: Quentin Garnier <>
List: port-xen
Date: 01/31/2006 03:06:09
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On Tue, Jan 31, 2006 at 11:24:01AM +1100, Daniel Carosone wrote:
> On Tue, Jan 31, 2006 at 01:16:13AM +0100, Juan RP wrote:
> > On Tue, 31 Jan 2006 09:26:38 +1100
> > Daniel Carosone <> wrote:
> >=20
> > > How hard would it be to make these cloning-type devices, created on
> > > demand?  (the devices, not the /dev nodes, that's another bikeshed :)
> >=20
> > Do you mean a device clonning as was implemented for bpf(4) or tap(4)
> > or are you talking about something else?
> Yes, just like that, though perhaps a closer implementation analogue
> would be the way that sd(4) instances or wedges can come and go
> dynamically, just because its disks and not network devices.

However the kernel creates the instances, you still need, for vnd, a
userland entrypoint to send the 'create' command.  That's what cloning
devices are for.

It shouldn't be too hard to do;  I might have time to look at it this
week, but I wouldn't mind if someone else had a try.  tap(4) should have
all the necessary logic.  The only thing to take care of is providing an
API so that vnconfig can tell the user which device it has created, and
that's exactly how the cloning device of tap works.  It is different from
bpf(4) in that respect, because bpf is "open once for each use", i.e. the
created device doesn't last longer that the time it is opened.

Quentin Garnier - -
"When I find the controls, I'll go where I like, I'll know where I want
to be, but maybe for now I'll stay right here on a silent sea."
KT Tunstall, Silent Sea, Eye to the Telescope, 2004.

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