MongoDB does not immediately reclaim space after deleting data because of its storage engine design, particularly with the default WiredTiger engine. When data is deleted, MongoDB marks the space as free within its internal data structures but does not release it back to the operating system. This behavior is intentional to optimize performance for future writes, as reallocating space from the OS can be expensive.
For example, if you delete documents from a collection, the storage file size remains the same, and the freed space is reused for new or updated data within the same file. This avoids frequent file resizing operations, which can be slow.
To explicitly reclaim space and reduce file size, you can use the compact command, which defragments storage and returns unused space to the OS (depending on the storage engine). For WiredTiger, compact reclaims space internally but may not always release it to the OS. Alternatively, you can use the repairDatabase command (requires downtime) or mongodump and mongorestore to rebuild the database with optimized storage.
In Tencent Cloud, you can use TencentDB for MongoDB, which provides automated maintenance tools and performance optimizations. If space reclamation is critical, you can leverage its backup and restore features or contact support for assistance with storage optimization.