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    SubjectRe: [autofs] [RFC] Towards a Modern Autofs
    raven@themaw.net wrote:

    >On Mon, 12 Jan 2004, Mike Waychison wrote:
    >
    >
    >
    >>>Transparency of an autofs filesystem (as I'm calling it) is the situation
    >>>where, given a map
    >>>
    >>>/usr /man1 server:/usr/man1
    >>> /man2 server:/usr/man2
    >>>
    >>>where the filesystem /usr contains, say a directory lib, that needs to be
    >>>available while also seeing the automounted directories.
    >>>
    >>>
    >>>
    >>>
    >>>
    >>I see. This requires direct mount triggers to do properly. Trying to
    >>do it with some sort of passthrough to the underlying filesystem is a
    >>nightmare waiting to happen..
    >>
    >>
    >>
    >
    >So what are we saying here?
    >
    >We install triggers at /usr/man1 and /usr/man2.
    >Then suppose the map had a nobrowse option.
    >
    >
    This is a direct map. The browse / nobrowse options do not apply to
    direct maps.

    >Does the trigger also take care of hiding man1 and man2?
    >
    >
    >
    No. man1 and man2 appear as directories to anyone doing an lstat on
    them. Traversing *into* them will cause filesystems to be mounted on
    them. This appears to be similar to browsing of an indirect map at
    first, however it is a different beast. With indirect maps, we are
    given the right to cover up /usr to help us detects stats and traversals
    into its sub-directories. With direct entries, we don't have these
    leisure. Everything in /usr most be accessible at all times.

    Your need for 'transparency' comes from the fact that you convert direct
    maps into indirect maps, which require the covering of /usr.

    >Is there some definition of these triggers?
    >
    >
    >
    This question is up in the air.

    I propose using a magic filesystem, whose root dentry has a follow_link
    callback defined. When somebody walks into the filesystem, the
    follow_link is called, which does the mount onto a different dentry, and
    then forwards the original caller to the new vfsmount/dentry pair.

    HPA and Viro believe this is better done in the VFS layer directly by
    using special vfsmounts without super_blocks. The path walking code
    would be modified to know of these 'traps' or 'triggers' natively.

    Which solution is best is left as an exercise.

    --
    Mike Waychison
    Sun Microsystems, Inc.
    1 (650) 352-5299 voice
    1 (416) 202-8336 voice
    mailto: Michael.Waychison@Sun.COM
    http://www.sun.com

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    NOTICE: The opinions expressed in this email are held by me,
    and may not represent the views of Sun Microsystems, Inc.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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