Re: [RFC] Union mount readdir support in glibc

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Tue, Mar 11, 2008 at 01:09:29AM -0700, Roland McGrath wrote:
> It seems very unlikely you'd come up with a version of this plan that we'd
> find acceptable in glibc.  readdir does buffering, sometimes entry format
> conversion, and it can skip dummy entries.  That's it.  It's not going to
> become a big hairy thing with all kinds of new state.  Sorry.

In the approach we are suggesting, at the minimum, glibc readdir would
have to maintain a unified cache of dirents with the knowlege of
whiteouts (DT_WHT). Would that be too much ?

> 
> This really is the kernel filesystem's problem.  It just doesn't make sense
> to expect userland to implement half of your directory semantics for you.
> What are you going to do when you want to export a union directory to NFS?
> readdir is a filesystem operation.  You're implementing a filesystem.

Not really. In Union Mount, most of the unification support is done at
VFS layer with some support from filesystems (for things like
whiteouts). It is Unionfs which implements a new filesystem to achieve
unification. Unification is not purely a kernel filesystem's problem, it
involves both VFS and FS.

> 
> Exposing DT_WHT entries may be useful as a user feature.  (BSD had unions
> with whiteouts years ago, and their ls et al have options to let you see
> and operate on whiteouts explicitly so users can make sense of strange
> situations with unions.)  But even for that, we'd have to consider the
> compatibility issues.

AFAIK, even BSD implements duplicate elimination and whiteout
suppression in the userland.

Thanks for your comments.

Regards,
Bharata.