Re: [RFC PATCH 1/4] Union mount documentation.

[Jan Blunck <jblunck@suse.de>]

On Tue, 19 Jun 2007 22:59:51 -0700, Arjan van de Ven wrote:

> first of all I'm happy to see that people are still working on unionfs;
> I'd love to have functionality like this show up in Linux.

This has nothing to do with unionfs. This is about doing a VFS based
approach to union mounts. Unification is a name-based construct so it
belongs into VFS and not into a separate file system.

> I'll not claim to have any VFS knowledge whatsoever, but I was just
> wondering what happens in the following scenario:
> 
> FS A is mounted twice, in /mnt/A and /mnt/union
> 
> FS B is mounted twice, in /mnt/B and as topmost union mount
> on /mnt/union
> 
> lets for simplicity say both filesystems are entirely empty
> 
> user does on FS A: 
> mkdir  /mnt/A/somedir
> touch /mnt/A/somedir/somefile
> 
> and then 2 things happen in parallel
> 1) touch /mnt/B/somefile
> 2) mv /mnt/union/somedir /mnt/union/somefile
> 
> since the underlying FS for 2) is FS A... how will this work out locking
> wise? Will the VS lock the union directory only? Or will this operate
> only on the underlying FS? How is dcache consistency guaranteed for
> scenarios like this?

Mounting a file system twice is bad in the first place. This should be
done by using bind mounts and bind a mounted file system into a union.
After that the normal locking rules apply (and hopefully work ;).

Jan