---
id: bf9c4b82-5485-43b3-8c68-9a6b7dc22263
---

# MongoDB Oplog Growth Impacting Replication
---

This incident type refers to a situation where the MongoDB Oplog, a capped collection in MongoDB that records all the write operations to a database, grows rapidly and impacts the replication process. This can occur for various reasons, such as high write volume or inefficient oplog retention settings. It can result in replication lag and failures, impacting the availability and consistency of the database.

### Parameters
```shell
export MONGODB_DATABASE_NAME="PLACEHOLDER"

export NEW_SIZE_IN_MEGABYTES="PLACEHOLDER"

export RETENTION_HOURS="PLACEHOLDER"

export OPLOG_SIZE="PLACEHOLDER"

export MONGODB_CONFIG_SERVER_ADDRESS="PLACEHOLDER"

export MONGODB_SHARD_ADDRESS="PLACEHOLDER"

export MONGODB_COLLECTION_NAME="PLACEHOLDER"

export SHARD_KEY_FIELD="PLACEHOLDER"
```

## Debug

### Check the current oplog size and usage
```shell
mongo ${MONGODB_DATABASE_NAME} --eval "printjson(db.oplog.rs.stats())"
```

### Check the current replication status and lag
```shell
mongo ${DB_NAME} --eval "printjson(rs.status())"
```

### Check the current and historical oplog growth rate
```shell
mongo ${DB_NAME} --eval "printjson(db.oplog.rs.stats(1024*1024).maxSize)"
```

### Check the oplog retention settings
```shell
mongo ${DB_NAME} --eval "printjson(db.getMongo().getDB('local').oplog.rs.options())"
```

### Optimize the oplog retention settings to reduce oplog growth
```shell
mongo ${DB_NAME} --eval "db.getMongo().getDB('local').runCommand({replSetResizeOplog: 1, size: ${NEW_SIZE_IN_MEGABYTES}})"
```

## Repair

### Increase the size of the oplog and adjust the retention settings to accommodate the write volume and replication needs of the system.
```shell


#!/bin/bash



# Set the ${DB_NAME} and ${OPLOG_SIZE} parameters

DB_NAME=${DB_NAME}

OPLOG_SIZE=${OPLOG_SIZE}



# Connect to the MongoDB instance and switch to the ${DB_NAME} database

mongo --eval "use $DB_NAME"



# Increase the oplog size to ${OPLOG_SIZE} MB

mongo --eval "db.adminCommand({replSetResizeOplog: 1, size: $OPLOG_SIZE})"



# Adjust the oplog retention settings to ${RETENTION_HOURS} hours

mongo --eval "db.getSiblingDB('local').setProfilingLevel(0)"

mongo --eval "db.getSiblingDB('local').createCollection('oplog.rs', {capped: true, size: $((OPLOG_SIZE*2*1024*1024)), max: 10000})"

mongo --eval "db.getSiblingDB('local').runCommand({collMod: 'oplog.rs', usePowerOf2Sizes: true})"

mongo --eval "db.getSiblingDB('local').runCommand({collMod: 'oplog.rs', indexDetails: {expireAfterSeconds: $(RETENTION_HOURS*60*60)}})"


```

### Implement sharding or horizontal scaling to distribute the load across multiple nodes and reduce the write volume on individual nodes.
```shell


#!/bin/bash



# Set variables

MONGODB_CONFIG=${MONGODB_CONFIG_SERVER_ADDRESS}

MONGODB_SHARD=${MONGODB_SHARD_ADDRESS}

MONGODB_DATABASE=${MONGODB_DATABASE_NAME}

MONGODB_COLLECTION=${MONGODB_COLLECTION_NAME}

SHARD_KEY=${SHARD_KEY_FIELD}



# Enable sharding for the database

mongo --host $MONGODB_CONFIG --eval "sh.enableSharding('$MONGODB_DATABASE')"



# Create the sharded collection

mongo --host $MONGODB_CONFIG --eval "sh.shardCollection('$MONGODB_DATABASE.$MONGODB_COLLECTION', {'$SHARD_KEY': 'hashed'})"



# Add the shard to the cluster

mongo --host $MONGODB_CONFIG --eval "sh.addShard('$MONGODB_SHARD')"


```

This incident type refers to a situation where the MongoDB Oplog, a capped collection in MongoDB that records all the write operations to a database, grows rapidly and impacts the replication process. This can occur for various reasons, such as high write volume or inefficient oplog retention settings. It can result in replication lag and failures, impacting the availability and consistency of the database.


```shell
export MONGODB_DATABASE_NAME="PLACEHOLDER"

export NEW_SIZE_IN_MEGABYTES="PLACEHOLDER"

export RETENTION_HOURS="PLACEHOLDER"

export OPLOG_SIZE="PLACEHOLDER"

export MONGODB_CONFIG_SERVER_ADDRESS="PLACEHOLDER"

export MONGODB_SHARD_ADDRESS="PLACEHOLDER"

export MONGODB_COLLECTION_NAME="PLACEHOLDER"

export SHARD_KEY_FIELD="PLACEHOLDER"
```


### Check the current oplog size and usage

```shell
mongo ${MONGODB_DATABASE_NAME} --eval "printjson(db.oplog.rs.stats())"
```

### Check the current replication status and lag

```shell
mongo ${DB_NAME} --eval "printjson(rs.status())"
```

### Check the current and historical oplog growth rate

```shell
mongo ${DB_NAME} --eval "printjson(db.oplog.rs.stats(1024*1024).maxSize)"
```

### Check the oplog retention settings

```shell
mongo ${DB_NAME} --eval "printjson(db.getMongo().getDB('local').oplog.rs.options())"
```

### Optimize the oplog retention settings to reduce oplog growth

```shell
mongo ${DB_NAME} --eval "db.getMongo().getDB('local').runCommand({replSetResizeOplog: 1, size: ${NEW_SIZE_IN_MEGABYTES}})"
```


### Increase the size of the oplog and adjust the retention settings to accommodate the write volume and replication needs of the system.

```shell


#!/bin/bash



# Set the ${DB_NAME} and ${OPLOG_SIZE} parameters

DB_NAME=${DB_NAME}

OPLOG_SIZE=${OPLOG_SIZE}



# Connect to the MongoDB instance and switch to the ${DB_NAME} database

mongo --eval "use $DB_NAME"



# Increase the oplog size to ${OPLOG_SIZE} MB

mongo --eval "db.adminCommand({replSetResizeOplog: 1, size: $OPLOG_SIZE})"



# Adjust the oplog retention settings to ${RETENTION_HOURS} hours

mongo --eval "db.getSiblingDB('local').setProfilingLevel(0)"

mongo --eval "db.getSiblingDB('local').createCollection('oplog.rs', {capped: true, size: $((OPLOG_SIZE*2*1024*1024)), max: 10000})"

mongo --eval "db.getSiblingDB('local').runCommand({collMod: 'oplog.rs', usePowerOf2Sizes: true})"

mongo --eval "db.getSiblingDB('local').runCommand({collMod: 'oplog.rs', indexDetails: {expireAfterSeconds: $(RETENTION_HOURS*60*60)}})"


```

### Implement sharding or horizontal scaling to distribute the load across multiple nodes and reduce the write volume on individual nodes.

```shell


#!/bin/bash



# Set variables

MONGODB_CONFIG=${MONGODB_CONFIG_SERVER_ADDRESS}

MONGODB_SHARD=${MONGODB_SHARD_ADDRESS}

MONGODB_DATABASE=${MONGODB_DATABASE_NAME}

MONGODB_COLLECTION=${MONGODB_COLLECTION_NAME}

SHARD_KEY=${SHARD_KEY_FIELD}



# Enable sharding for the database

mongo --host $MONGODB_CONFIG --eval "sh.enableSharding('$MONGODB_DATABASE')"



# Create the sharded collection

mongo --host $MONGODB_CONFIG --eval "sh.shardCollection('$MONGODB_DATABASE.$MONGODB_COLLECTION', {'$SHARD_KEY': 'hashed'})"



# Add the shard to the cluster

mongo --host $MONGODB_CONFIG --eval "sh.addShard('$MONGODB_SHARD')"


```


MongoDB Oplog Growth Impacting Replication

Overview

Parameters

Debug

Check the current oplog size and usage

Check the current replication status and lag

Check the current and historical oplog growth rate

Check the oplog retention settings

Optimize the oplog retention settings to reduce oplog growth

Repair

Increase the size of the oplog and adjust the retention settings to accommodate the write volume and replication needs of the system.

Implement sharding or horizontal scaling to distribute the load across multiple nodes and reduce the write volume on individual nodes.

Learn more

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