MongoDB switching to WireTiger storage engine

We were already running in production with a mongodb cluster of 3 nodes (replicated) which were running out of diskspace, each node having access to a 750 GB drive at 77% usage. The obvious solution was to expand the diskspace, but at the same time I wanted to be more efficient with the disk space usage itself.

Previously we were using the storage engine callled MMAPv1 which had no support for compression and I wanted to switch over to the WireTiger storage engine which does have support for compression options.

Here I describe the strategy I used :

Since my mongoDB cluster was replicated, I was able to take down one node at a time to perform the switch over to WiredTiger. Once I was finished with one node, I could bring it back up, and take down the next node, and so on until all nodes were upgraded. By doing it this way, there was no downtime whatsoever from the perspective of the user.

For each node I did the following:

• Shutdown the mongod service
• Moved the mongo data folder, which in my case was /var/lib/mongo, to another volume attached storage for backup purposes in case procedure fails.
• Recreate the mongo data folder, in my case /var/lib/mongo and assign the appropriate permissions: chown mongod:mongod /var/lib/mongo
• Modify the /etc/mongod.conf configuration file to include the following: storageEngine=wiredTiger
• Restart mongod service
• Check wiredTiger is configured correctly using the mongo command-line:

 db.serverStatus().storageEngine  
 { "name" : "wiredTiger", "supportsCommittedReads" : true }  

Now that the node is back up and running, replication will happen in the background. If you head over to your primary mongo node, and type rs.status() and you should see a status of STARTUP2.
Once the node has replicated successfully, repeat the same procedure for the next node.

Reference:

https://docs.mongodb.com/v3.0/release-notes/3.0-upgrade/?_ga=1.86531032.1131483509.1428671022#change-replica-set-storage-engine-to-wiredtiger

https://askubuntu.com/questions/643252/how-to-migrate-mongodb-2-6-to-3-0-with-wiredtiger