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SubjectRe: 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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