In the past I was kind of depending on the service times reported for db file parallel write waits via AWRs. The reason for looking at db file parallel write service times was to help in determining if there were write I/O service time issues.
I brought up looking at db file parallel write times in a conversation with Kevin Closson and he gave me his “Two Cents Worth” on the topic and suggested I read Frits Hooglands blog accessed via: https://fritshoogland.wordpress.com/2013/09/06/oracle-io-on-linux-database-writer-io-and-wait-events/
So, I read Frits' blog which was nicely written!
The bottom line, from Frits’ blog, for sync I/O: basically, db file parallel write waits are not timed at all. And I/Os are executed sequentially.
The bottom line, from Frit’s blog, for async I/O: db file parallel write waits shows the response time for the minimal number of I/O requests from the I/O completion queue. For example, via io_submit system call, 32 async write I/Os are submitted. Via io_getevents system call, it tries to reap/read at least a minimum number of I/O events (ie. 2) from the completion queue, but not necessarily the 32 write requests. So if it reads/reaps completion times of only 2 of the write I/Os, what is the value of the I/O timing provided? Well, really none.
Yet another lesson learned.