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    SubjectRe: [rfc] Near-constant time directory index for Ext2
    On Tue, 20 Feb 2001, Linus Torvalds wrote:
    > In article <01022020011905.18944@gimli>,
    > Daniel Phillips <> wrote:
    > >Earlier this month a runaway installation script decided to mail all its
    > >problems to root. After a couple of hours the script aborted, having
    > >created 65535 entries in Postfix's maildrop directory. Removing those
    > >files took an awfully long time. The problem is that Ext2 does each
    > >directory access using a simple, linear search though the entire
    > >directory file, resulting in n**2 behaviour to create/delete n files.
    > >It's about time we fixed that.
    > Interesting.
    > However, if you're playing with the directory structure, please consider
    > getting rid of the "struct buffer_head"-centricity, and using the page
    > cache instead. The page cache has much nicer caching semantics, and
    > looking up data in the page cache is much faster because it never needs
    > to do the "virtual->physical" translation.

    Oh yes, I was planning on it. I started with the buffers version
    for two main reasons version: 1) it's simple and solid and 2) it
    provides the basis for a backport to 2.2 - after the 2.4/2.5 version is
    complete of course.

    > Talk to Al Viro about this - he's already posted patches to move the
    > regular ext2 directory tree into the page cache, and they weren't
    > applied to 2.4.x only because there was no great feeling of "we _must_
    > do this for correctness".
    > I see that you already considered this issue, but I wanted to bring it
    > up again simply because something like this certainly looks like a
    > potential candidate for 2.5.x, but I will _refuse_ to add code that
    > increases our reliance of "struct buffer_head" as a caching entity. So
    > I'd rather see the page cache conversion happen sooner rather than
    > later...

    You are preaching to the converted.

    > Also, just out of interest: if you've already been worrying about
    > hashes, what's the verdict on just using the native dentry hash value
    > directly? It has other constraints (_really_ low latency and absolutely
    > performance critical to calculate for the common case, which is not
    > needing a real lookup at all), but maybe it is good enough? And if not,
    > and you have done some statistics on it, I'd love to hear about it ;)

    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.

    Let the hash races begin.

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