Deleting documents in MongoDB does not reclaim the disk space used by it. If we have 10 GB of disk space used by MongoDB and we delete all documents, we will still be using 10 GB. What happens under the hood is that MongoDB will mark these documents as deleted and may use the space to store new documents.

This results in our disk having space that is not used, yet not freed up for the operating system. If we want to claim it back, we can use compact() to reclaim unused space:

> db.books.compact()

Or alternatively, we can start the mongod server with the option --repair.

A better option is to enable compression, available from version 3.0 and only with the WiredTiger storage engine. We can use the snappy or zlib algorithm to compress our document size. This will, again, not prevent storage holes, but if we are tight on disk space, it is preferable to the heavy operational route of repair and compact.

Storage compression uses less disk space at the expense of CPU usage, but this trade-off is mostly worth it.