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

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Message-ID: <20070412023712.GA8175@lazybastard.org>
Date:	Thu, 12 Apr 2007 04:37:12 +0200
From:	Jörn Engel <joern@...ybastard.org>
To:	Neil Brown <neilb@...e.de>
Cc:	Theodore Tso <tytso@....edu>, "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 Thu, 12 April 2007 11:46:41 +1000, Neil Brown wrote:
> 
> I could argue that nfs came before ext3+dirindex, so ext3 should have
> been designed to work properly with NFS.  You could argue that fixing
> it in nfsd fixes it for all filesystems.  But I'm not sure either of
> those arguments are likely to be at all convincing...

Caring about a non-ext3 filesystem, I sure would like an nfs solution as
well. :)

> Hmmm. I wonder.  Which is more likely?
>   - That two 64bit hashes from some set are the same
>   - or that 65536 48bit hashes from a set of equal size are the same.

The former.  Each bit going from hash strength to collision chain length
reduces the likelihood of an overflow.  In the extreme case of a 0bit
hash and 64bit collision chain, you need 2^64 entries compared to 2^32
for the other extreme.

However, the collision chain gives me quite a bit of headache.  One
would have to store each entry's position on the chain, deal with older
entries getting deleted, newer entries getting removed, etc.  All this
requires a lot of complicated code that basically never gets tested in
the wild.

Just settling for a 64bit hash and returning -EEXIST when someone causes
a collision an creat() sounds more appealing.  Directories with 4
billion entries will cause problems, but that is hardly news to anyone.

Jörn

-- 
Fantasy is more important than knowledge. Knowledge is limited,
while fantasy embraces the whole world.
-- Albert Einstein
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