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    SubjectRe: Eureka! (was Re: UTF-8 and case-insensitivity)

    I'm probably just thicker than a complete set of superman comics, and
    probably haven't had enough coffee this morning, but I'm still trying
    to understand exactly how much this is going to gain us.

    If I understand it, your suggestion gives us:

    - a way of telling if a directory is fully cached in the dcache
    - a way of scanning that full cache with whatever braindead
    comparison algorithm we want

    At first I didn't understand the scanning part at all, because I
    didn't realise that you could scan just the dentries associated with a
    single directory. Al was kind enough to correct me on that.

    What your proposal doesn't give us is case-insensitive indexing into
    the dcache. The reason the dcache is such a great thing in Linux is
    that it is indexed by name, so you rarely do any scanning at all, and
    even the case where you have never seen the name before we avoid
    scanning because fast filesystems also use a "indexed by name"
    scheme. Now maybe I'm just over-obsessed about this scanning stuff and
    I'd need some profiles to see how much it would cost (although the
    cost as the directory size gets really large seems obvious).

    The really interesting part of your proposal is that it opens up the
    possibility of a coherence mechanism between a cache that is indexed
    by some windows like scheme and the real dcache. If those two bits
    could be used by the windows_braindead module to determine if its own
    separately indexed cache was current then we'd really be getting

    If we didn't do the separate cache at all, then your proposal still
    should hugely reduce the number of times we ask the filesystem for a
    list of files in the directory, although as those calls are already
    cached at the block device level what I suppose it does is move the
    cache up a level. I don't have a clear idea of how much faster it is
    to do this scanning in the dcache versus in the filesystem in the
    hot-cache case, so I am not clear on how much this wins us. I'm
    prepared to believe it could be quite significant though.

    I really need more coffee-and-think time on this, plus maybe some
    quick and dirty profiling tests to see what the various costs are

    While I'm here I should point out that I'm thinking of the 2.7/2.8
    kernel (or even 3.0) for any change, not 2.6. Maybe thats obvious
    anyway, but the corresponding userspace changes in Samba definately
    won't be happening in Samba 3.0, so this is a Samba 4.0 thing, which
    is a fair way off. This means we've got plenty of time to try some
    experiments and see what schemes really help.

    Cheers, Tridge
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