Deleting documents from an ApsaraDB for MongoDB instance does not free disk space immediately. This topic describes three methods to reclaim unused disk space. You can use these methods independently or in combination.

Background

ApsaraDB for MongoDB uses WiredTiger as the default storage engine starting from MongoDB 3.2. The storage.directoryPerDB:true parameter is enabled, so each database is stored as a separate folder on disk, and each collection is a separate file within that folder. For more information, see storage.directoryPerDB.

When you run the delete command, WiredTiger marks the storage space as free for reuse instead of returning it to the operating system. This behavior is similar to how MySQL handles deleted rows. Disk usage does not decrease after document deletion.

To reclaim disk space, use one of the following methods.

Method 1: Drop unused indexes, collections, or databases

Dropping an index, collection, or database deletes the underlying files immediately and frees disk space right away.

Identify unused indexes, collections, or databases.
Run the appropriate command:
- dropIndexes to remove unused indexes
- drop to remove unused collections
- dropDatabase to remove unused databases

Disk space is released as soon as the command completes.

Method 2: Compact collections to reclaim free space

After deleting a large number of documents, run the compact command to reclaim the freed space within a collection. First, check how much free space exists.

Check free space

Run the collStats command and compare the StorageSize and freeStorageSize fields. A large freeStorageSize relative to StorageSize indicates significant reclaimable space.

Run the compact command

Run the compact command on each collection that has significant free space. For detailed instructions, see Defragment a disk to improve disk usage.

Important

Run the compact command on each node separately. In a replica set, compact one node at a time. On a primary node, add force: true to the command, for example: db.runCommand({compact: "collectionName", force: true}).

Warning

In versions earlier than MongoDB 4.4, the compact command blocks all other operations. Proceed with caution.

Method 3: Copy documents to a new collection (offline only)

Instead of running compact, copy all documents to a new collection and rename it. This approach works only in offline scenarios because it requires exclusive access to the data.

Use the cursor.forEach method to iterate through the source collection and insert each document into a new collection.
Drop the original collection to free disk space.
Rename the new collection to match the original collection name.

Note

This method requires application downtime. Use Method 2 (compact) for online scenarios.

ApsaraDB for MongoDB:Release disk space of an ApsaraDB for MongoDB instance