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In "The Art of Unix Programming", on the topic of The Terminfo Database I read:

If you look in the terminfo directory, you'll see subdirectories named by single printable characters. Under each of these are the entries for each terminal type that has a name beginning with that letter. The goal of this organization was to avoid having to do a linear search of a very large directory; under more modern Unix file systems, which represent directories with B-trees or other structures optimized for fast lookup, the subdirectories won't be necessary.

I wonder if there are widespread (i.e. production ready) filesystems with this quality.

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up vote 4 down vote accepted

There are several, e.g. ext4, Microsoft's NTFS, Apple's HDF+, or the up and coming btrfs that use B-Trees. There are also HDF and Reiser4 which use B*-Trees, a more densely packed version of B-Tree.

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ext4 actually uses a modified version of B-tree called H-tree. This functionality is also available in ext3 through the dir_index option. –  ire_and_curses Sep 29 '12 at 18:34

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