linux-kernel - Re: If not readdir() then what?

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Message-Id: <1176242364.309.69.camel@heimdal.trondhjem.org>
Date:	Tue, 10 Apr 2007 17:59:24 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Neil Brown <neilb@...e.de>
Cc:	Theodore Tso <tytso@....edu>,
	Jörn Engel <joern@...ybastard.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ulrich Drepper <drepper@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: If not readdir() then what?

On Wed, 2007-04-11 at 07:37 +1000, Neil Brown wrote:
> On Tuesday April 10, trond.myklebust@....uio.no wrote:
> > On Wed, 2007-04-11 at 07:12 +1000, Neil Brown wrote:
> > > 
> > > Is there something that makes that interface problematic?
> > 
> > File deletions...
> 
> How are they a problem ?
> 
> There are only two ways to organise a directory.
> 1/ Unsorted linear list of entries in which no repacking is ever done.
> 2/ Some data structure using an ordered search key that is based on
>   the filename (e.g. a B-tree with a search key that is a hash of the
>   filename). 
> 
> In the first case, you just use a fixed opaque cookie for location in
> a directory.
> In the second you use the filename.  If the file has been deleted,
> that shouldn't stop you finding the place where it would have been in
> the overall sort order.

I beg to differ. RAMFS is an instance of a filesystem which uses an
unsorted linear list of entries which is automatically repacked when you
delete a file.

You may also have filesystems which are only partially sorted. The
dcache is for instance organised as a hashtable with multiple entries
per bucket...

Cheers
  Trond

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