Bgwriter strategies

I ran some DBT-2 tests to compare different bgwriter strategies:

http://community.enterprisedb.com/bgwriter/

imola-336 was run with minimal bgwriter settings, so that most writes 
are done by backends. imola-337 was patched with an implementation of 
Tom's bgwriter idea, trying to aggressively keep all pages with 
usage_count=0 clean. Imola-340 was with a patch along the lines of 
Itagaki's original patch, ensuring that there's as many clean pages in 
front of the clock head as were consumed by backends since last bgwriter 
iteration.

All test runs were also patched to count the # of buffer allocations, 
and # of buffer flushes performed by bgwriter and backends. Here's those 
results (I hope the intendation gets through properly):
        imola-336    imola-337    imola-340
writes by checkpoint      38302          30410          39529
writes by bgwriter     350113        2205782        1418672
writes by backends    1834333         265755         787633
writes total        2222748        2501947        2245834
allocations        2683170        2657896        2699974

It looks like Tom's idea is not a winner; it leads to more writes than 
necessary. But the OS caches the writes, so let's look at the actual I/O 
performed to be sure, from iostat:

http://community.enterprisedb.com/bgwriter/writes-336-337-340.jpg

The graph shows that on imola-337, there was indeed more write traffic 
than on the other two test runs.

On imola-340, there's still a significant amount of backend writes. I'm 
still not sure what we should be aiming at. Is 0 backend writes our goal?

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com