On Tue, 2006-12-05 at 10:07 +0000, Christoph Hellwig wrote:
> > ...and we have pointed out how nicely this ignores the realities of
> > current caching models. There is no need for a readdirplus() system
> > call. There may be a need for a caching barrier, but AFAICS that is all.
>
> I think Andreas mentioned that it is useful for clustered filesystems
> that can avoid additional roundtrips this way. That alone might now
> be enough reason for API additions, though. The again statlite and
> readdirplus really are the most sane bits of these proposals as they
> fit nicely into the existing set of APIs. The filehandle idiocy on
> the other hand is way of into crackpipe land.
They provide no benefits whatsoever for the two most commonly used
networked filesystems NFS and CIFS. As far as they are concerned, the
only new thing added by readdirplus() is the caching barrier semantics.
I don't see why you would want to add that into a generic syscall like
readdir() though: it is
a) networked filesystem specific. The mask stuff etc adds no
value whatsoever to actual "posix" filesystems. In fact it is
telling the kernel that it can violate posix semantics.
b) quite unnatural to impose caching semantics on all the
directory _entries_ using a syscall that refers to the directory
itself (see the explanations by both myself and Peter Staubach
of the synchronisation difficulties). Consider in particular
that it is quite possible for directory contents to change in
between readdirplus calls.
i.e. the "strict posix caching model' is pretty much impossible
to implement on something like NFS or CIFS using these
semantics. Why then even bother to have "masks" to tell you when
it is OK to violate said strict model.
c) Says nothing about what should happen to non-stat() metadata
such as ACL information and other extended attributes (for
example future selinux context info). You would think that the
'ls -l' application would care about this.
Trond
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