Hi,

The procedure describing how to change the config servers to WiredTiger should consider using a different port number for each time a mongod config server process is started with an empty dbpath.

This situation applies for three points of the procedure: step 6 to 7 (3rd config server), step 13 to 14 (2nd config server), step 18 to 19 (1st config server).

In particular, the critical path is step 18 to step 19, when this is done for the 1st config server of the cluster. During this time, two things will happen:

• Differ config warnings due to different config databases on the config servers
• As soon as the 1st config server is up, the mongos will use it and it will not be able to see the config database (no chunks, no collections, etc) until the restore is completed. This can cause the mongos to not be able to query the data existing in the shards.

This situation can be easily avoided by making sure that we use a different port number when starting the mongod config server process with the WiredTiger storage engine using an empty dbpath - until the restore is completed. At this point, the process can be safely restarted with its original port.