Re: Shadowfs - some notes

[Moritz Schulte]

Roland McGrath <roland@gnu.org> writes:

Hello,

>> The first question was: If shadowfs supports having an explicitely
>> specified writable filesystem[0], shadowfs might has to create whole
>> directory hierarchies - how can this be done? (Or should it be done at
>> all?.)
>
> In a shadowfs that supports writing at all, this is most certainly the
> behavior it should have.  Anything else just doesn't make sense as a user.
> Incidentally, this is what BSD's "union" mount type does.

Ah. It's nice to see that that's an similar approach to unionfs's.

>> Example: settrans /shadow /hurd/shadowfs --writable /a /b
>> (which explicitely specifies /a as the only writable filesystem).
>
> Just as an aside, might I suggest supporting relative pathnames taken
> as starting at the underlying node, so:
>
>       settrans /shadow /hurd/shadowfs --writable . /b
>
> is the same as your example, but uses the directory underneath the
> translator at /shadow in place of /a.  On BSD, this is done with:
>
>       mount -t union -o -b /b /shadow

Perhaps there should be an command line option, which specifies that
the next given filesystem should be relative to the translated node
and not to the cwd?

>> 1) it would mean that in every looked up virtual directory node in
>>    shadowfs, the according path would have to be stored. This is not
>>    nice. (An alternative to storing the path in every node would be to
>>    automatically create the directories on the writable filesystem as
>>    they are successfully looked up. Then the creation of whole
>>    directory hierarchies wouldn't be needed.)
>
> From a little experimentation, creating all the directories on
> read-only lookups seems to be what BSD does. Seems a little iffy to
> me, but if it's ok for them I guess it's not so bad.

I think it's a good way. It makes everything much easier. Instead of
caring about creating whole directory hierarchies _at once_, shadowfs
tries to keep the writable filesystem 'in sync' with the lookups done.

        moritz
-- 
Moritz Schulte <moritz@chaosdorf.de> http://www.chaosdorf.de/moritz/
"In short: just say NO TO DRUGS, and maybe you won't end up like
the Hurd people." - Linus Torvalds.
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