MongoDB documents take up a specified amount of space on disk. If we perform an update that increases the size of a document this may end up being moved out of sequence to the end of the storage block, creating a hole in storage, resulting in increased execution times for this update, and possibly missing it from running queries. The compact operation will defragment space resulting in less space being used.

compact can also take a paddingFactor argument as follows:

> db.runCommand ( { compact: '<collection>', paddingFactor: 2.0 } )

paddingFactor is preallocated space in each document that ranges from 1.0 (no padding, the default value) to 4.0 for calculated padding of 300 bytes for each 100 bytes of document space needed when we initially insert it.

Adding padding can help alleviate the problem of updates moving documents around, at the expense of more disk space being needed for each document when created.