linux-kernel - Re: [RFC] Union Mount: Readdir approaches

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Message-ID: <20070910034624.GA6415@in.ibm.com>
Date:	Mon, 10 Sep 2007 09:16:24 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	"Josef 'Jeff' Sipek" <jsipek@...sunysb.edu>
Cc:	hooanon05@...oo.co.jp, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hch@...radead.org,
	Jan Blunck <jblunck@...e.de>
Subject: Re: [RFC] Union Mount: Readdir approaches

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@...oo.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.
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