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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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