> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

# replSetResizeOplog (database command)

## Definition

`replSetResizeOplog` also supports specifying the minimum number of hours to preserve an oplog entry.

**Changed in version 5.0**

To set the `replSetOplog` size in [`mongosh`](https://www.mongodb.com/docs/mongodb-shell.md#mongodb-binary-bin.mongosh), use the `Double()` constructor.

`replSetResizeOplog` enables you to resize the oplog or its minimum retention period dynamically without restarting the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) process.

You must run this command against the `admin` database.

## Compatibility

This command is available in deployments hosted in the following environments:

**Important:**

This command is not supported in MongoDB Atlas clusters. For information on Atlas support for all commands, see [Unsupported Commands.](https://www.mongodb.com/docs/atlas/unsupported-commands/)

- [MongoDB Enterprise](https://www.mongodb.com/docs/manual/administration/install-enterprise.md#std-label-install-mdb-enterprise): The subscription-based, self-managed version of MongoDB

- [MongoDB Community](https://www.mongodb.com/docs/manual/administration/install-community.md#std-label-install-mdb-community-edition): The source-available, free-to-use, and self-managed version of MongoDB

**Note:**

To resize the oplog in Atlas, see [Set Minimum Oplog Window.](https://www.mongodb.com/docs/atlas/cluster-additional-settings.md#std-label-set-oplog-min-window)

## Syntax

The command has the following form:

```javascript
db.adminCommand(
   {
     replSetResizeOplog: <int>,
     size: <double>,
     minRetentionHours: <double>
   }
 )
```

## Command Fields

The command takes the following fields:

| Field | Type | Description |
| --- | --- | --- |
| `replSetResizeOplog` | int | Set to `1`. |
| `size` | double | The maximum size of the oplog in megabytes. The minimum size you can specify is 990 megabytes.; The maximum size you can specify is 1 petabytes.; Explicitly cast the `size` as a double in [`mongosh`](https://www.mongodb.com/docs/mongodb-shell.md#mongodb-binary-bin.mongosh) with `Double()`. See [Change the Maximum Oplog Size.](https://www.mongodb.com/docs/manual/reference/command/replsetresizeoplog.md#std-label-oplog-ex-change-size) |
| `minRetentionHours` | double | *Optional*. The minimum number of hours to preserve an oplog entry, where decimal values represent the fractions of an hour. For example, a value of `1.5` represents one hour and thirty minutes. The value must be greater than or equal to `0`. A value of `0` indicates that the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) should truncate the oplog starting with the oldest entries to maintain the configured maximum oplog size. A [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) configured with `minRetentionHours` only removes an oplog entry *if*: The oplog has reached the maximum configured size, *and*; The oplog entry is older that the configured number of hours based on the host system clock. To check the currently configured minimum oplog retention period, see the [`oplogTruncation.oplogMinRetentionHours`](https://www.mongodb.com/docs/manual/reference/command/serverstatus.md#mongodb-serverstatus-serverstatus.oplogTruncation.oplogMinRetentionHours) in the output of the [`serverStatus`](https://www.mongodb.com/docs/manual/reference/command/serverstatus.md#mongodb-dbcommand-dbcmd.serverStatus) command. |

**See also:**

- [`--oplogSize`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#std-option-mongod.--oplogSize) / [`replication.oplogSizeMB`](https://www.mongodb.com/docs/manual/reference/configuration-options.md#mongodb-setting-replication.oplogSizeMB)

- [`--oplogMinRetentionHours`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#std-option-mongod.--oplogMinRetentionHours) / [`storage.oplogMinRetentionHours`](https://www.mongodb.com/docs/manual/reference/configuration-options.md#mongodb-setting-storage.oplogMinRetentionHours)

## Behavior

You can use `replSetResizeOplog` on [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) instances running with the [Wired Tiger storage engine](https://www.mongodb.com/docs/manual/core/wiredtiger.md#std-label-storage-wiredtiger) or the [in-memory storage engine.](https://www.mongodb.com/docs/manual/core/inmemory.md#std-label-storage-inmemory)

See the [Change the Oplog Size of Self-Managed Replica Set Members](https://www.mongodb.com/docs/manual/tutorial/change-oplog-size.md#std-label-tutorial-change-oplog-size) tutorial for a procedure on using `replSetResizeOplog` command to resize the oplog.

The oplog can grow past its configured size limit to avoid deleting the [`majority commit point`.](https://www.mongodb.com/docs/manual/reference/command/replsetgetstatus.md#mongodb-data-replSetGetStatus.optimes.lastCommittedOpTime)

You cannot drop the `local.oplog.rs` collection. For more information on this restriction, see [Oplog Collection Behavior.](https://www.mongodb.com/docs/manual/core/replica-set-oplog.md#std-label-oplog-coll-behavior)

`replSetResizeOplog` overrides the maximum oplog size or minimum oplog retention period set at startup by:

- [`replication.oplogSizeMB`](https://www.mongodb.com/docs/manual/reference/configuration-options.md#mongodb-setting-replication.oplogSizeMB) / [`--oplogSizeMB`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#std-option-mongod.--oplogSize), and

- [`storage.oplogMinRetentionHours`](https://www.mongodb.com/docs/manual/reference/configuration-options.md#mongodb-setting-storage.oplogMinRetentionHours) / [`--oplogMinRetentionHours`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#std-option-mongod.--oplogMinRetentionHours) respectively.

The new oplog size persists after a server restart, unless you use:

- [`storage.oplogMinRetentionHours`](https://www.mongodb.com/docs/manual/reference/configuration-options.md#mongodb-setting-storage.oplogMinRetentionHours), or

- [`--oplogMinRetentionHours`.](https://www.mongodb.com/docs/manual/reference/program/mongod.md#std-option-mongod.--oplogMinRetentionHours)

**Important:**

Reducing the maximum oplog size results in truncation of the oldest oplog entries until the oplog reaches the new configured size.

Similarly, reducing the minimum oplog retention period results in truncation of oplog entries older that the specified period *if* the oplog has exceeded the maximum configured size.

Oplog truncation due to reduced oplog size or retention period can result in unexpected behavior from clients still reading those oplog entries, including:

- Open [change streams](https://www.mongodb.com/docs/manual/changestreams.md#std-label-changeStreams) may become invalidated

- Secondaries which have not replicated those oplog entries may require [resynchronization.](https://www.mongodb.com/docs/manual/core/replica-set-sync.md#std-label-replica-set-initial-sync)

- Backups using [`mongodump`](https://www.mongodb.com/docs/database-tools/mongodump.md#mongodb-binary-bin.mongodump) with [`--oplog`](https://www.mongodb.com/docs/database-tools/mongodump.md#std-option-mongodump.--oplog) against the member may not capture entries prior to truncation.

### Minimum Oplog Retention Period

A [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) has the following behavior when configured with a minimum oplog retention period:

- The oplog can grow without constraint so as to retain oplog entries for the configured number of hours. This may result in reduction or exhaustion of system disk space due to a combination of high write volume and large retention period.

- If the oplog grows beyond its maximum size, the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) may continue to hold that disk space even if the oplog returns to its maximum size *or* is configured for a smaller maximum size. See [Reducing Oplog Size Does Not Immediately Return Disk Space.](https://www.mongodb.com/docs/manual/reference/command/replsetresizeoplog.md#std-label-replSetResizeOplog-cmd-compact)

- The [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) compares the system wall clock to an oplog entry creation [wall clock time](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-wall-clock-time) when enforcing oplog entry retention. Clock drift between cluster components may result in unexpected oplog retention behavior. See [Clock Synchronization](https://www.mongodb.com/docs/manual/administration/production-notes.md#std-label-production-notes-clock-synchronization) for more information on clock synchronization across cluster members.

### `replSetResizeOplog` Does Not Replicate To Other Members

Changing the oplog size or minimum oplog retention period of a given replica set member with `replSetResizeOplog` does not change the oplog size of any other member in the replica set. You must run `replSetResizeOplog` on each replica set member in your cluster to change the oplog size or minimum retention period for all members.

### Reducing Oplog Size Does Not Immediately Return Disk Space

Reducing the oplog size does not immediately reclaim that disk space. This includes oplog size reduction due to truncation of oplog events older than of the [minimum oplog retention period.](https://www.mongodb.com/docs/manual/reference/command/replsetresizeoplog.md#std-label-replSetResizeOplog-cmd-minimum-retention)

To immediately free unused disk space after reducing the oplog size, run [`compact`](https://www.mongodb.com/docs/manual/reference/command/compact.md#mongodb-dbcommand-dbcmd.compact) against the `oplog.rs` collection in the `local` database during a maintenance period. `compact` blocks all operations on the database it runs against. Running `compact` against `oplog.rs` therefore prevents oplog synchronization. For a procedure on resizing the oplog and compacting `oplog.rs`, see [Change the Oplog Size of Self-Managed Replica Set Members.](https://www.mongodb.com/docs/manual/tutorial/change-oplog-size.md)

### Resource Locking

`replSetResizeOplog` takes an exclusive (W) lock on the [`oplog`](https://www.mongodb.com/docs/manual/reference/local-database.md#mongodb-data-local.oplog.rs) and blocks other operations on the collection until it finishes.

For more information on locking in MongoDB, see [FAQ: Concurrency.](https://www.mongodb.com/docs/manual/faq/concurrency.md)

## Examples

### Change the Maximum Oplog Size

Use the [`db.collection.stats()`](https://www.mongodb.com/docs/manual/reference/method/db.collection.stats.md#mongodb-method-db.collection.stats) [`mongosh`](https://www.mongodb.com/docs/mongodb-shell.md#mongodb-binary-bin.mongosh) method to display the current maximum oplog size, `maxSize`, in megabytes. For example:

```javascript
db.getSiblingDB("local").oplog.rs.stats(1024*1024).maxSize
```

The above command returns the oplog size of this member in megabytes:

```javascript
990
```

The following command uses `replSetResizeOplog` to change the oplog size of this member to 16384 megabytes:

```javascript
db.adminCommand({ "replSetResizeOplog": 1, size: Double(16384)})
```

To verify the new oplog size, rerun the [`db.collection.stats()`](https://www.mongodb.com/docs/manual/reference/method/db.collection.stats.md#mongodb-method-db.collection.stats) method:

```javascript
db.getSiblingDB("local").oplog.rs.stats(1024*1024).maxSize
```

The above command returns:

```javascript
"maxSize": Long("16834")
```

**Warning:**

Reducing the size of the oplog in a node removes data from it. This may cause replica members syncing with that node to become stale. To resync those members, see [Resync a Member of a Self-Managed Replica Set.](https://www.mongodb.com/docs/manual/tutorial/resync-replica-set-member.md)

### Change the Minimum Oplog Retention Period

1. Connect [`mongosh`](https://www.mongodb.com/docs/mongodb-shell.md#mongodb-binary-bin.mongosh) to the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) replica set member.

2. *Optional*. Use the [`db.serverStatus()`](https://www.mongodb.com/docs/manual/reference/method/db.serverstatus.md#mongodb-method-db.serverStatus) command to verify the current minimum oplog retention value as [`oplogTruncation.oplogMinRetentionHours`:](https://www.mongodb.com/docs/manual/reference/command/serverstatus.md#mongodb-serverstatus-serverstatus.oplogTruncation.oplogMinRetentionHours)

   ```javascript
   db.getSiblingDB("admin").serverStatus().oplogTruncation.oplogMinRetentionHours
   ```

   The command returns the currently configured minimum oplog retention period for the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod). For example:

   ```javascript
   1.5
   ```

   If the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) has no minimum oplog retention period, the operation returns an empty result.

3. Use the `replSetResizeOplog` command to modify the configured minimum oplog retention period. For example, the following sets the minimum oplog retention period to `2` hours:

   ```javascript
   db.adminCommand({
     "replSetResizeOplog" : 1,
     "minRetentionHours" : 2
   })
   ```