Re: [RFC] Union Mount: Readdir approaches

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

On Fri, Sep 07, 2007 at 01:39:41PM -0400, Josef 'Jeff' Sipek wrote:
> On Fri, Sep 07, 2007 at 01:28:55PM +0530, Bharata B Rao wrote:
> > On Fri, Sep 07, 2007 at 04:31:26PM +0900, hooanon05@yahoo.co.jp wrote:
> > > 
> > > When the first readdir is issued:
> > > - call vfs_readdir for every underlying opened dir (file) object.
> > > - store every entry to either the hash table for the result or the
> > >   whiteout, when the same-named entry didn't exist in the tables.
> > > - to improvement the performance, the allocated memory for the hash
> > >   tables are managed in a pointer array. and the elements are
> > >   concatinated logically by the pointer.
> > > - the pointer for the result-table, the version, and the currect jiffies
> > >   are set to vdir, which is a cache in an inode.
> > > - all cache are copied to a member in a file object.
> > > - the index of the cache memory block and the offset in an array is
> > >   handled as the seek position.
> > 
> > Ok, interesting approach. So you define the seek behaviour on your
> > directory cache rather than allowing the underlying filesystems to
> > interpret the seek. I guess we can do something similar with Union
> > Mounts also.
> 
> Unless I missunderstood something, Unionfs uses the same approach. Even

But in the version of unionfs present in -mm, lseek on directories is
still limited in functionality as it allows seeking to only the
beginning and to the current position.

> Unionfs's ODF branch does the same thing. The major difference is that we
> keep the cache in a file on a disk.

And as Erez explained, it is ODF which is allowing you to have a
complete lseek behaviour.

Regards,
Bharata.