# Migrate from mmap to WiredTiger Storage Engine

Starting with the major release 4.x of Rocket.Chat, MongoDB has to be set up with a [**WiredTiger**](https://www.mongodb.com/docs/manual/core/wiredtiger/) storage engine rather than the deprecated *mmapv1* one. This is mandatory, if you plan to upgrade to one of the future Rocket.Chat versions have to be prepared before initiating the application upgrade.

This guide helps you migrate your existing `mmap` based MongoDB instance to WiredTiger manually.

## Requirements

* MongoDB instance with *mmap* storage engine.

## Quick usage

Following is an overview of the migration steps:

1. Stop running Rocket.Chat.
2. Create a database dump from the *mmapv1* MongoDB (also to have a backup in place).
3. Stop *mmapv1* MongoDB service, drop existing data files, and start up with WiredTiger.
4. Import the dump into the new WiredTiger MongoDB.
5. Start Rocket.Chat.

## Detailed usage

Here are the detailed steps of the migration process:

1. Stop Rocket.Chat service to ensure a consistent database dump:

   ```bash
   systemctl stop rocketchat
   ```
2. Create a database dump from the current *mmapv1* MongoDB:

   ```bash
   mongodump --archive=~/mmapdump.gz --gzip
   ```
3. Stop MongoDB service:

   ```bash
   systemctl stop mongod
   ```
4. Delete the *mmapv1* based data files of your existing MongoDB:

   ```bash
   rm -rf /var/lib/mongodb/*
   ```
5. Adjust MongoDB configuration to make use of *wiredTiger* storage engine:

   ```
   vi /etc/mongod.conf
   ```

   ```bash
   [...]
   engine: wiredTiger
   [...]
   ```
6. Start MongoDB service again:

   ```bash
   systemctl start mongod
   ```
7. If running with a Replica-Set in your mongo.conf initialize replica set

   ```bash
   mongo --eval 'rs.initiate()'
   ```
8. Import dump back into (*wiredTiger*) MongoDB:

   ```bash
   mongorestore --drop --archive=~/mmapdump.gz --gzip --noIndexRestore
   ```
9. Repair databases and rebuild indices:

   ```bash
   mongo --eval 'db.repairDatabase()'
   ```
10. Start Rocket.Chat service:

    ```bash
    systemctl start rocketchat
    ```