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

Jamie Lokier
2001-02-21T23:27:16Z

Martin Mares wrote:
> Hello!
>
> > True. Note too, though, that on a filesystem (which we are, after all,
> > talking about), if you assume a large linear space you have to create a
> > file, which means you need to multiply the cost of all random-access
> > operations with O(log n).
>
> One could avoid this, but it would mean designing the whole filesystem in a
> completely different way -- merge all directories to a single gigantic
> hash table and use (directory ID,file name) as a key, but we were originally
> talking about extending ext2, so such massive changes are out of question
> and your log n access argument is right.

A gigantic hash table has serious problems with non-locality of
reference. Basically any regular access pattern you started with is
destroyed. This is a problem with pageable RAM, let alone disks with
millisecond seek times.

-- Jamie