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SubjectRe: [RFC PATCH 1/4] Union mount documentation.
On Wed, 2007-06-20 at 11:21 +0530, Bharata B Rao wrote:
> From: Bharata B Rao <>
> Subject: Union mount documentation.

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.

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?

Arjan van de Ven

if you want to mail me at work (you don't), use arjan (at)
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