bug: File empty value problem caused by `mmap IO`

[qishenonly]

As shown in the following figure, using mmap can result in a large number of empty values in the file, which needs to be resolved quickly.But that doesn't happen with fileio.

[ZhaoShuaiUp]

Beacause you didn't call db.close() function

[qishenonly]

All right, that's my problem.