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Date: Wed, 2 Apr 2014 18:17:06 +0200
From: Michael Olbrich <m.olbrich@pengutronix.de>
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Subject: Re: [ptxdist] machine-id
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Hi,

On Wed, Apr 02, 2014 at 02:25:29PM +0000, Gieseler, Christian wrote:
> > On Mon, Mar 31, 2014 at 01:13:28PM +0000, Gieseler, Christian wrote:
> > > while thinking how to preserve the existing machine-id after an
> > > complete update I had I look how this is generated. In the rc-once.d
> > > folder there is a script generating an id and systemd also has the
> > > capability to generate one and obviously does check for a valid
> > > machine-id and regenerates one in case of an invalid/non exisiting one.
> > > Looking into the dependencys  the ptxdist machine-id script is selectd
> > > with systemd. Is the existance of the ptxdist machine-id necessary if
> > > systemd takes care of the machine id?
> > > And finaly coming back to the original question what is the best way
> > > to preserve the machine-id? Patching Systemd to change the hardcoded
> > > path? Or are there other smarter solutions?
> > 
> > The machine-id is read very early, so even if you change the path in systemd,
> > so the path must be in the rootfs and is probably overwritten during a
> > complete update.
> > You could explicitly save the machine-id before an update and copy it back
> > afterwards.
> > If you have an initrd then you could bind mount  your machine-id to
> > /etc/machine-id before starting systemd.
> 
> Ok, thats clear and easy to implement.
> 
> Can you explain why one needs the script /etc/rc.once.d/machine-id if the
> machine id is generated by systemd?

If the rootfs is read-only, then systemd will bind-mount a file on a tmpfs
to /etc/machine-id and fill it with a uuid. Without
/etc/rc.once.d/machine-id this would happen on every boot.
When /etc/rc.once.d/machine-id runs the rootfs is temporarily writeable and
the machine-id is copied to the rootfs to make it permanent.

Michael

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