Re: is it possible to disable the 3-way merge feature?

["Colin Walters" <walters verbum org>]

On Mon, Dec 16, 2019, at 4:22 PM, Davis Roman via ostree-list wrote:
> Hi,
>
> I've often wondered if it is possible to completely disable ostree's 3 
> way merge feature.

Would absolutely be happy to offer that as an option!  We'd need to do a bit of design; probably best done 
via a Github issue.

But at least one question before we do that: In this scenario, could each machine have a read-only /etc?  You 
probably want to at least have a unique /etc/machine-id per machine - but it's thereafter read-only?

In Fedora CoreOS for example we use Ignition and OSTree, and Ignition is "run exactly once" provisioning - 
while we don't make /etc read-only by default thereafter, I'd like to support doing so. 

A tricky aspect of this is if you do want the "receive changes to defaults" semantic that OSTree provides 
(e.g. if a config file /etc/foo.conf appears in a new version, you want it, if it goes away, it should go 
away on devices unless it was edited, etc.).

If the main concern is about the cost to do the merge, using a filesystem with reflinks will help (which is 
mostly XFS and BTRFS).  That said, we could also investigate using overlayfs for /etc.  That would give us 
the "only content written is new files" semantic.

Another possibility is to say if /etc is read-only, we hardlink files there just like we do for /usr.  That 
would be simpler, but at the cost of wanting to make changes to /etc more dangerous (vi /etc/foo.conf would 
change all copies).