Since these two upgrade checks perform collection scans to scan all the documents, it may take a while to complete the check if the data size is big.

It would be nice to add this warning in the documents for these two commands, so that users can be aware of the time required to perform the upgrade check.

• http://docs.mongodb.org/manual/reference/method/db.upgradeCheckAllDBs/
• http://docs.mongodb.org/manual/reference/method/db.upgradeCheck/