All of your questions are answered in tied hashed and deleting keys and their valeus (sic), a recent thread on this behavior of DBM files.

To summarize: It is an optimization that DBM files do not shrink when keys are deleted. Shuffling the bits around on disk every time a key is deleted would be too slow. Instead, the space is reused the next time a key is inserted. Read the aforementioned thread for more details.

In reply to Re: DBM problem by chipmunk
in thread DBM problem by frechettes

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