Re: Implementing mount_union(8) into vfs (for -o union)?

[Alistair Crooks <agc%pkgsrc.org@localhost>]

On Sat, Jan 28, 2012 at 09:47:16PM +0200, Alan Barrett wrote:
> On Sat, 28 Jan 2012, Julian Fagir wrote:
> >I've just been trying to mount a tmpfs over a read-only root 
> >file system.  Unfortunately, this won't work just by mounting a 
> >tmpfs with option union over the root file system. You'd have to 
> >create a tmpfs, and mount that one with mount_union(8) over the 
> >root file system, which is again not possible.
> 
> I read your message twice and I still don't know what you mean. 
> Could you give examples of the commands that you use, and the 
> errors.

An example of using unionfs, rather than union dirs, is in
src/distrib/embedded/mkimage - which unfortunately has too many tmpfs
incantations, but does work well for usermode.  More should happen in
this area in future.  I think that I used mount_union as union dirs
didn't handle whiteouts properly, but it's been a while, and I didn't
write things down from that period.

One thing I did find was that some log files are opened O_WRONLY|O_APPEND,
and so need to be copied manually into the upper layer first or the
corresponding upper-layer entry does not created.

Regards,
Alistair