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[Xen-devel] persistent or "steady-state" style disks


I have a requirement (and a limited timeframe) to make a Windows 7 VM domain roll back to a fixed state on restart, shutdown etc.
Something like a Kiosk Mode.  I think this is a very needed part of the core of xen and this functionality does 
not appear to be part of xen already.  So I've been trying various ways to make this work.

Has anyone already implemented something like this successfully before?

What I've been experimenting unsuccessfully with currently is the following:

1) Add hooks into the blktap2 code to roll back a VHD (currently based on the name ending in -persist.vhd).  
However, it's not working out as it should.

2) Second attempt was to write an external script and call it periodically.
The idea was to call vhd-snapshot on the parent disk to a new disk (temp name).  Then do an atomic rename on existing vhd
and unlink the old vhd.  When it restarts you would think it would open up the new snapshot.  Unfortunately, a reboot seems to
detect that the file is different and so it's not re-started as desired but instead the domain is shutdown.

I don't like either of the above methods.  They are hacks in my opinion but I'm not currently familiar enough with xen internals.  
Can someone propose a method and/or suggest where I could hook into the xen tools to implement it?

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