Mongo shell cannot use commands write mode, degrading to compatibility mode

You are using a new mongo (>= 2.6) shell to connect a older version mongod (2.4). The new mongo shell now integrates the write concern directly into the method (insert, update) rather than with a separate getLastError command, which is not supported by older mongod. The new shell then decide to downgrade to be compatible.

Solution: You can simply ignore this warning or try to use the correct mongo shell version.