Horton

Re: Spread checkpoint sync

Greg Smith <greg@2ndquadrant.com>

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From: Greg Smith <greg@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Jeff Janes <jeff.janes@gmail.com>, Ron Mayer <rm_pg@cheapcomplexdevices.com>, Josh Berkus <josh@agliodbs.com>, pgsql-hackers@postgresql.org
Date: 2011-01-17T03:13:59Z
Lists: pgsql-hackers

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  1. Add new buffers_backend_fsync field to pg_stat_bgwriter.

I have finished a first run of benchmarking the current 9.1 code at 
various sizes.  See http://www.2ndquadrant.us/pgbench-results/index.htm 
for many details.  The interesting stuff is in Test Set 3, near the 
bottom.  That's the first one that includes buffer_backend_fsync data.  
This iall on ext3 so far, but is using a newer 2.6.32 kernel, the one 
from Ubuntu 10.04.

The results are classic Linux in 2010:  latency pauses from checkpoint 
sync will easily leave the system at a dead halt for a minute, with the 
worst one observed this time dropping still for 108 seconds.  That one 
is weird, but these two are completely averge cases:

http://www.2ndquadrant.us/pgbench-results/210/index.html
http://www.2ndquadrant.us/pgbench-results/215/index.html

I think a helpful next step here would be to put Robert's fsync 
compaction patch into here and see if that helps.  There are enough 
backend syncs showing up in the difficult workloads (scale>=1000, 
clients >=32) that its impact should be obvious.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books