Re: [rfc] Near-constant time directory index for Ext2

Linus Torvalds
2001-02-21T00:23:30Z

On Tue, 20 Feb 2001, Daniel Phillips wrote:
>
> You mean full_name_hash? I will un-static it and try it. I should have
> some statistics tomorrow. I have a couple of simple metrics for
> measuring the effectiveness of the hash function: the uniformity of
> the hash space splitting (which in turn affects the average fullness
> of directory leaves) and speed.

I was more thinking about just using "dentry->d_name->hash" directly, and
not worrying about how that hash was computed. Yes, for ext2 it will have
the same value as "full_name_hash" - the difference really being that
d_hash has already been precomputed for you anyway.

> Let the hash races begin.

Note that dentry->d_name->hash is really quick (no extra computation), but
I'm not claiming that it has anything like a CRC quality. And it's
probably a bad idea to use it, because in theory at least the VFS layer
might decide to switch the hash function around. I'm more interested in
hearing whether it's a good hash, and maybe we could improve the VFS hash
enough that there's no reason to use anything else..

Linus