According to the OSG PR Display, the OSG runs and accounts for 420,000 jobs daily. That's 4.8 records a second added into MySQL JobUsageRecord table. Also, for each job, summary data is updated in the summary table. So that's at least 5 inserts/updates a second, but most likely many more. All the while, we are able to interactively query the database (well, a replicated one) using pages such as the UNL graphs.
We also keep track of transfers, but they are summarized and aggregated at the probe before reaching the database.
So, in summary, kudos to the gratia operations team.
(NOTE: I may have forgotten some optimizations that we use. But still, that's a lot of records)