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

["H. Peter Anvin" <hpa@transmeta.com>]

Daniel Phillips wrote:
> 
> Have you looked at the structure and algorithms I'm using?  I would not
> call this a hash table, nor is it a btree.  It's a 'hash-keyed
> uniform-depth tree'.  It never needs to be rehashed (though it might be
> worthwhile compacting it at some point).  It also never needs to be
> rebalanced - it's only two levels deep for up to 50 million files.
> 

I'm curious how you do that.  It seems each level would have to be 64K
large in order to do that, with a minimum disk space consumption of 128K
for a directory.  That seems extremely painful *except* in the case of
hysterically large directories, which tend to be the exception even on
filesystems where they occur.

I think I'd rather take the extra complexity and rebalancing cost of a
B-tree.

> This thing deserves a name of its own.  I call it an 'htree'.  The
> performance should speak for itself - 150 usec/create across 90,000
> files and still a few optmizations to go.
> 
> Random access runs at similar speeds too, it's not just taking advantage
> of a long sequence of insertions into the same directory.
> 
> BTW, the discussion in this thread has been very interesting, it just
> isn't entirely relevant to my patch :-)

	-hpa

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