persistent or "steady-state" style disks

[Matthew Hook <matthew.hook@otoy.com>]

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Hi,

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?

Regards,
Matthew

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