Messages in this thread | | | From | Jan Blunck <> | Subject | Re: [RFC PATCH 1/4] Union mount documentation. | Date | Wed, 20 Jun 2007 07:29:55 +0000 (UTC) |
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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
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