A user raised an issue yesterday with questions about the meta-data upgrade procedure when upgrading from 2.2 to 2.4. The documentation is here. While most of the documentation is straightforward and self-explanatory, this section might need a revisit:

If any index on the config database is {v:0}, you should drop that index using db.collection.dropIndex(), and then rebuild it with db.collection.ensureIndex(). This does not apply to indexes on other databases in the cluster. If you need to upgrade the _id index to {v:1} use the db.collection.reIndex().

Specifically:
1. Is it necessary to drop & recreate indexes instead of just running db.collection.reIndex() on all collections which contain

indexes?
2. Explicitly mention that customers should do this once on the mongos which would then propagate to all configsvrs. The customer thought he had to do it once on each configsvr by connecting directly to it => this is very error prone.

Please let me know if this makes sense.

Thanks, Angshuman