mongodb - Why and what case to issue mongodb repair command

Question

I am using Mongodb 2.4.8 on a 64 bit machiene with 3 servers as replicaSet, for which i have currently disbaled journaling on my development box .

Durabilty is not so important for our Application , so the reason i have disabled Journaling Option .

I see that there is only one advantage of journaling , that is in case of an unclean shutdown we dont have to issue a repair command as journaling will take care of it .

To produce this unclean shutdown i killed mongo replica process using kill -9 Mongo process Id , i just removed mongo locks and restarted the mongo primary , secondary and the arbitery servers , everything started fine .

My question is that , when i should we issue the repair command actually (as removing locks and restart works )

Please excuse if the question is too dumb , as i wanted to know the risk of disbaling journaling under production .

Answer 1

The repairDatabase command checks your whole database for corrupted data and discards that data so the rest becomes usable again.

This can become necessary after an unclear shutdown. In your case the shutdown didn't appear to corrupt any data (or maybe it did, but it didn't become apparent yet because the data in question wasn't accessed yet). But that doesn't mean that this will always be the case. Was your database actually doing anything at that moment? When the database is idle or only performing read-operations, there is usually not much to worry about. But when it is currently in the middle of a large write-operation, a sudden shutdown without journaling can be much more troublesome.

Another scenario where a database could be corrupted and repairDatabase could help is a physical malfunction of the storage medium or a corruption of the underlying filesystem.

Important note regarding replica-sets: When you have a replica-set, and only one node is corrupted, then you should rather remove that node and rebuild it from the other members of the replica-set. RepairDatabase will destroy any corrupted data. Restoring from a replica-set will not.