R12: ARCH wait on SENDREQ

Days after our R12 upgrade our wait event alert showed that "ARCH wait on SENDREQ" was quite high, so further investigation showed 9 different sessions in the wait event within gv$session with all of them having the program set like "oracle@server (ARC0)".

The next day, this is what our wait event alert looked like:

  Waits                                                                                      Wait Time
  gc buffer busy release                                                               6515.7246
  gc buffer busy acquire                                                               6243.267722
  ARCH wait on SENDREQ                                                      4103.947898
  Backup: MML write backup piece                                           3537.058172
  log file sync                                                                              3190.876444
  SQL*Net message from dblink                                                2942.456672
  SQL*Net break/reset to client                                                 1443.085918
  gc current block busy                                                               1434.390776
  FAL archive wait 1 sec for REOPEN minimum                         1156.983756

Referencing the official documentation again for the definition of "buffer busy waits" and "log file sync" helped us to pin not only the ARCH activity, but everything else going on the next day, down to the SLA conversion process that was still being running post-deployment.  What we were seeing was the archiver and archive logs processing from the amount of redo/undo being created in the system, which was why gc (global cache) was so high in the system and the "FAL archive wait" was being generated.