How to migrate MongoDB 2.6 to 3.0 with WiredTiger

MongoDB 3.0 introduces a new storage engine called wiredTiger which results in greatly reduced memory and disk space usage.

Since my DB is currently at ~20 million objects and using up ~70GB of RAM, this update comes in at just the right time to postpone a hardware update.

How do you migrate an existing installation of MongoDB 2.6 to 3.0 and at the same time get the benefits of wiredTiger?

The Documentation refers to options which result in startup errors which prevent MongoDB from starting up. Also, file locations don't match the ones in Ubuntu (Server 14.04 LTS).

2 Answers

In default installations, the configuration file is at /etc/mongod.conf. What the MongoDB docs don't mention is that when migrating to WiredTiger we also need to update the configuration file to the new YAML format introduced in 2.6.
As far as I can tell the engine option is only available in the new configuration format.
Migrating from the old storage engine consists in creating a database dump, shutting down mongodb, changing settings and then importing the dump into the new storage engine.

1. Create a backup. Seriously. We need a database dump which we'll then import to the new database engine:

   mongodump -d db_name /backup/path/ 

2. Stop the mongodb service

   sudo service mongod stop 

3. Move data from the current location to somewhere else (MongoDB will not startup if the data directory contains files generated by the old storage engine).

   sudo mv /var/lib/mongodb /var/lib/mongodb_26/

4. Upgrade MongoDB to version 3.0 (from ):

   sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
   echo "deb "$(lsb_release -sc)"/mongodb-org/3.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list
   sudo apt-get update
   sudo apt-get install mongodb-org mongodb-org-server mongodb-org-shell mongodb-org-mongos mongodb-org-tools 

5. Convert the configuration file from old (pre 2.6) to the current YAML format. The bare minimum is:

   storage: dbPath: "/var/lib/mongodb" engine: wiredTiger
   systemLog: destination: file path: "/var/log/mongodb/mongod.log" logAppend: true
   net: bindIp: 127.0.0.1 port: 27017 # Enable the HTTP interface (Defaults to port 28017). http: enabled: false 

   Make sure no lines in the old format remain, or MongoDB won't start.

   The full documentation for the configurtion file is at:

6. Optionally make a backup of the log:

   sudo mv /var/log/mongodb/mongod.log /var/log/mongodb/mongod_26.log

7. Restart mongodb

   sudo service mongod start

8. Load the backup to convert data to new storage engine

   mongorestore /backup/location

After checking that all your data is ok, you can delete the directory with the old data format

sudo rm -r /var/lib/mongodb_26/

Note that for replica sets and sharded clusters there are some aditional steps:

Using the old configuration file format, I had success with:

storageEngine=wiredTiger