reference regarding write load during different stages of checkpoint

I'm hoping to find a document I once read about the write load.

As best I can recall, it looked something like this:

. at beginning of (spread) checkpoint, larger than average write load to
pg_wal/, due to full_page_writes;
. during most of checkpoint, decreasing WAL due to FPW,
. towards end of checkpoint, increased writes to table data base/, due to
fsync();
. assuming the next checkpoint doesn't start immediately, quiescent period, due
to clean OS buffers;

This isn't very important, but I hadn't seen that described before, and
I think there was more detail than I can remember.

I've been hoping for awhile to run across it and not able to find it. It
probably dates back to 8.3/9.0 days and maybe disappeared.

Does anyone know what I'm talking about or where I can find it?

Thanks,
Justin