[Pkg-xen-devel] Bug#452721: irt: Bug#452721 notes from explorations

[Elliott Mitchell]

Even though there hasn't been any discussion recently, bug #452721 is
very much still of major concern to me.

First issue is how to parse domain configuration files.  Reason being a
foo.cfg file might have the configuration 'name = "bar"'.  This would
also let the script retrieve the UUID if that has been set.

Turns out while Python in domain configuration files isn't supportted,
the syntax is still a proper subset of the Python language.  This makes
Python the ideal programming language for a replacement script.  Only
weakness is being able to have full Python syntax in configuration files
might make the task simpler.

Presently I hope to convince the Xen core to allow full Python in domain
configuration files, but no news on that front so far.  This would mean
/etc/default/xendomains would need to change to match Python syntax.


My thinking for adding to domain configuration files would be something
along these lines:

init = {
	'tool': 'xendomains-ng',
	'version': 0,
	'order': 9,
	'startwait': 60,
	'stopaction': 'save',
}

Mainly a Python dictionary holding key values.  Thought being the 'tool'
and 'version' values, is to hope for some form of compatibility if such
scripts were to become common.

My thinking is 'order' would indicate sequence.  Domains with higher
order get started first (same order would nominally allow parallel
start).  If a domain.cfg file didn't define order then its order is 0.

'startwait' would tell the script to wait that long before starting
subsquent domains.

'stopaction' would allow different actions if the machine was to stop.
The 3 options which come to mind are 'stop' (shutdown), 'save' (save to
specified storage location), and 'migrate'.


If full Python doesn't become available, this might take the format:

init = 'tool=xendomains-ng,version=0,order=9,startwait=60,stopaction=save'

Not needing to parse the string though does make one's life simpler.


Other concerns include:
Sometimes you may want to take a distinct action during stop.  Ie if
you're doing restarts for kernel updates, you'll want to override and
have domains reboot.

It may be handier to have distinct options for 'restart'.  Full restarts
can follow proper order, or could simply involve bouncing domains based
on order.  Notably with HVM domains and Qemu updates, you could do:

order 0 down, order 1 down, order 9 down, order 9 up, order 2 up, order 0 up

Or you could do:

order 9 down, order 9 up, order 1 down, order 1 up, order 0 down, order 0 up


I'm basically certain writing a new xendomains script in Python is the
way to go.  Now to get an answer as to whether full Python in domain
configuration files could be reenabled.


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