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

    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?


    Greetings,
    Arjan van de Ven

    --
    if you want to mail me at work (you don't), use arjan (at) linux.intel.com
    Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org

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