fsync/lock is only supposed to prevent writes. But in a recent dba training class, we tried to do a backup on a slave by applying this and then using mongodump on it, only to discover that mongodump was blocked until the lock was released. Since mongodump only does queries, why is it blocked?

Understand that locking is not required in this case (unlike file copies), but it is reasonable to use it if a guarantee of self-consistency for the server is required.