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RE: [Xen-devel] [PATCH]: xl: use libuuid to generate random UUID's

To: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, "Gianni Tedesco (3P)" <gianni.tedesco@xxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH]: xl: use libuuid to generate random UUID's
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Date: Tue, 10 Aug 2010 17:06:01 +0100
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Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH]: xl: use libuuid to generate random UUID's
> > > VM uuids are not stable in xend nor xl.  Perhaps xl should hash the VM
> > > name, which is stable and supposedly unique.
> >
> > But they can be made to be stable and I suppose this is the point here.
> No, I don't think VM uuids can be made stable in xl.  If for no other
> reason than that localhost migration would be impossible.

Localhost migration is useful for testing but is not a normal mode of 
operation.  It's fine to temporarily break the stability of UUIDs during the 
localhost migration as long as it is cleaned up afterwards. The VMs will not 
actually be running at the same time. 

> I think the only way to have stable MAC addresses is to derive them
> from the VM name, or for the user to explicitly write a MAC address in
> the config file.

Deriving MAC addresses from the VM UUID is the best approach if the MAC hasn't 
been specified in the config file. Locally administered addresses are the only 
sane thing to use if random addresses are being generated -- there aren't 
enough bits for AUI addresses to be generated randomly without high probability 
of collision.


> Users who do the latter will find things continue to work no matter
> how we change the default scheme, and the default is already somewhat
> broken, so perhaps we should indeed consider changing it.
> Ian.

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