You're already doing what he was suggesting anyway - storing your logs somewhere other than your application database.
The disk contention issue could be a problem because writes are hard to scale, but depending on the regularity of logging and the indexing used (I'm assuming it's done by timestamp here) it's probably not actually the biggest of deals - you should end up mostly doing append-only writes. In oracle, for example, all that'll happen is that for each log transaction some stuff will get altered in memory (i.e. a block of the table will get appended to, and an append will occur on the index) and the roll forward log will get written to on disk. Every so often the table and index blocks will get written to disk, but that will be much less than once per transaction in an appending case.
That said, overall I'd still recommend having the logging done on a separate system, or at least having profiling set up on your log table/user/whatever such that logging can't eat your production server if someone decides to excessively ramp up the log levels.