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List:       opensolaris-xen-discuss
Subject:    Re: [xen-discuss] Final Xen SMF fast-track
From:       John Levon <john.levon () sun ! com>
Date:       2007-06-28 20:37:37
Message-ID: 20070628203737.GB4165 () barman ! uk ! sun ! com
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On Mon, Jan 22, 2007 at 10:55:26PM -0500, Matty wrote:

> >3.  Xen Store Daemon
> >
> >    /usr/lib/xenstored is a simple daemon implementing the database of
> >    configuration parameters for each virtual instance running, and has the
> >    FMRI:
> >
> >        svc:/system/xctl/store:default.
> >
> >    Due to significant Xen shortcomings, the daemon is not currently safely
> >    restartable; if it fails or is otherwise stopped, the Xen control tools
> >    as administered via xm will not work. See:
> >
> >        6447387 can't restart xenstored
> >
> >    It is dependent upon svc:/system/filesystem/local.
> 
> Can you extrapolate on the Xen shortcomings? The bug mentions that the
> xenstore clients need to be modified to fix these shortcomings, but
> doesn't give a whole lot of detail on what is actually wrong (or what
> needs to be fixed).

Now I can see this, I can reply :)

It boils down to a number of points (you'll need some familiarity with xenstore
to understand most of these):

 - all domains in xenstore are cleaned up on a start, instead of just dead ones

 - xenstore watches are not stored persistently

 - dom0 userspace xenstore users are not prepared to see a connection to
   xenstored die. The simplest 'fix' for this is to restart all dependent
   services when xenstored restarts.

 - xenstored's shared domain pages are not read/written in a transactional
   manner. In particular, we can 'ack' a request before having written a reply;
   if we restart in between, then we will lose the request and the domU will
   hang forever

We will not be modifying xenstore clients when it comes to domU clients, but
may for dom0 clients, depending on exactly how we fix things.

regards
john
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