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    SubjectRe: [autofs] [RFC] Towards a Modern Autofs
    On Thu, 8 Jan 2004, H. Peter Anvin wrote:

    > Ian Kent wrote:
    > >
    > > If wildcard map entries are not in autofs v3 then Jeremy implemented this
    > > in v4.
    > >
    > v3 has had wildcard map entries and substitutions for a very, very, very
    > long time... it was a v2 feature, in fact.
    > > And yes the host map is basically a program map and that's all. Worse, as
    > > pointed out in the paper it mounts everything under it. This is a source
    > > of stress for mount and umount. I have put in a fair bit of time on ugly
    > > hacks to work around this. This same problem is also evident in startup
    > > and shutdown for master maps with a good number of entries (~50 or more).
    > > A consequence of the current multiple daemon approach.
    > This is why one wants to implement a mount tree with "direct mount
    > pads"; which also means keeping some state in the daemon.
    > For example, let's say one has a mount tree like:
    > /foo server1:/export/foo \
    > /foo/bar server1:/export/bar \
    > /bar server2:/export/bar
    > ... then you actually have four diffenent filesystems involved: first,
    > some kind of "scaffolding" (this can be part of the autofs filesystem
    > itself or a ramfs) that hold the "foo" and "bar" directories, and then
    > foo, foo/bar, and bar.
    > Consider the following implementation: when one encounters the above,
    > the daemon stashes this away as an already-encountered map entry (in
    > case the map entries change, we don't want to be inconsistent), creates
    > a ramfs for the scaffolding, creates the "foo" and "bar" subdirectories
    > and mount-traps "foo" and "bar". Then it releases userspace. When it
    > encounters an access on "foo", it gets invoked again, looks it up in its
    > "partial mounts" state, then mounts "foo" and mount-traps "foo/bar",
    > then releases userspace.

    Umm. The cross filesystem problem again.

    This may sound a little silly but it may be able to be done using
    stackable filesystem methods (aka. Zadok et. al.). I'm thinking of an
    autofs filesystem stacked on a host filesystem. The dentrys corresponding
    to mount points marked in some way and the mount occuring under it, on top
    of the host filesystem. Yes I know it sounds ugly but maybe it's not.
    Maybe it's actually quite simple. I can't give an opinion yet as I'm still
    thinking it through and haven't done any feasibility. However, this
    approach would lend itself to providing autofs filesystem transparency. A
    requirement as yet not discussed.


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