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An HTree is a specialized version of a B-tree. They are constant depth of either one or two levels, have a high fanout factor, use a hash of the filename, and do not require balancing. Htree indexes are used in the ext3 and ext4 Linux filesystems, and were incorporated into the Linux kernel around 2.5.40.
Htree indexes are available in ext3 when the dir_index feature is enabled.
Htree indexes are turned on by default in ext4. This feature is implemented in Linux kernel 2.6.23.