I reviewed this patch. My question for you is: does it make sense to enable to reporting of write rate even when vacuum cost accounting is enabled? In my opinion it would be useful to do so. If you agree, please submit an updated patch.
-- Álvaro Herrera <alvherre(at)commandprompt(dot)com> The PostgreSQL Company - Command Prompt, Inc. PostgreSQL Replication, Consulting, Custom Development, 24x7 support