Is data truly removed from the disk immediately after a delete?

When you delete documents in MongoDB using commands like deleteOne() or deleteMany(), the data is not immediately erased from disk. Instead, MongoDB performs a logical deletion: the documents are removed from the collection and no longer appear in query results, but their underlying disk space is simply marked as 'available for reuse' by the WiredTiger storage engine . This behavior is by design, optimized for write performance, and means the physical file size on your filesystem typically remains unchanged after a delete operation.

MongoDB's default WiredTiger engine manages disk space through sophisticated data structures. When data is deleted, the space it occupied is added to an internal 'available list' of reusable blocks . Future insert or update operations may write new data into these now-available blocks. This copy-on-write approach avoids expensive disk operations and fragmentation, but it also means that the operating system sees the file as the same size until those blocks are actually overwritten or the file is truncated . The disk space is only physically released back to the operating system under specific conditions, such as when a block at the very end of the file becomes free and is truncated during a checkpoint .