MongoDB Migrating away from RocksDB to WiredTiger

I wasn't able to use wiredTiger upgrading MongoDB. However, at May 17 I uninstalled MongoDB 2.6 then installed MongoDB 3.0.3 . Immediate after the installation, I added storageEngine=wiredTiger on top of my mongod.conf file. Then I gave sudo service mongod start command and eventually I could.

Edit:

For fresh installed as directed by official documentation;

1. Open configuration file using sudo nano /etc/mongod.conf
2. Change the # engine line to engine: wiredTiger like the below
3. Run mongod using the command sudo service mongod start

.

# Where and how to store data.
storage:
  dbPath: /var/lib/mongodb
  journal:
    enabled: true
  engine: wiredTiger
#  mmapv1:
#  wiredTiger:

Edit:

If the current version is supported the wiredTiger;

1. Get the backup of the current database using mongodump command
2. Stop the mongod service using sudo service mongod stop command
3. Add storageEngine=wiredTiger text as the first line of mongod.conf file
4. Delete the all file on /var/lib/mongodb (or /data/db folder if used)
   [This is important. Because MongoDB cannot convert the current MMAP db files to wiredTiger format]
5. Start the mongod service using sudo service mongod start command
6. Restore the database from the backup using [mongorestore][2] command
7. wiredTiger is being used...

To migrate data between different storage engines in MongoDB 3.0 you will need to perform an initial sync to the new replica set node or use mongorestore to seed a new replica set with your data.

If you want to add a node with a new storage engine to a live production replica set, rs.add() is the correct approach. To ensure a smooth upgrade I would also upgrade all existing nodes to the latest version of MongoDB 3.0.x with MMAP storage (the default) before adding your new WiredTiger node(s).

The key information in your question (and comments) is:

• socket exceptions related to networking timeouts:

  I NETWORK  [rsSync] Socket recv() timeout  x.x.x.x:x
  I NETWORK  [rsSync] SocketException: remote: x.x.x.x:x error: 9001 socket exception [RECV_TIMEOUT] server [x.x.x.x:x]
  

• deploying on Azure, which has some known issues with timeouts

As per the MongoDB production notes on Azure, you should reduce the TCP keepalive setting to avoid network timeouts:

The TCP keepalive on the Azure load balancer is 240 seconds by default, which can cause it to silently drop connections if the TCP keepalive on your Azure systems is greater than this value. You should set tcp_keepalive_time to 120 to ameliorate this problem.

As further preparation for successful initial sync with WiredTiger you should review the MongoDB production notes for your operating system.

Since you mention using Linux, I would recommend checking:

• ulimits are appropriately set. WiredTiger creates more files than MMAP (separate data & index files per collection rather than per database) so will use more file handles than MMAP nodes in the same replica set. If you don't increase the ulimits from the default you may find the initial data transfer completes but initial sync fails at the index build stage with a fatal error like Too many open files.

• the WiredTiger data directory is on a volume using the XFS file system. As at the time of writing, there are known performance issues (such as throughput stalls) with EXT4.

The production notes in the MongoDB manual are updated regularly based on issues and experiences reported by MongoDB users, so should be part of your preflight checklist for production O/S deployments or upgrades.