Message-ID: <4B6FDD26.9010106@2ndquadrant.com>
Date: Mon, 08 Feb 2010 04:45:10 -0500
From: Greg Smith <greg@2ndquadrant.com>
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To: pgsql-performance@postgresql.org
Subject: Linux I/O tuning:  CFQ vs. deadline
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Recently I've made a number of unsubstantiated claims that the deadline 
scheduler on Linux does bad things compared to CFQ when running 
real-world mixed I/O database tests.  Unfortunately every time I do one 
of these I end up unable to release the results due to client 
confidentiality issues.  However, I do keep an eye out for people who 
run into the same issues in public benchmarks, and I just found one:  
http://insights.oetiker.ch/linux/fsopbench/

The problem analyzed in the "Deadline considered harmful" section looks 
exactly like what I run into:  deadline just does some bad things when 
the I/O workload gets complicated.  And the conclusion reached there, 
"the deadline scheduler did not have advantages in any of our test 
cases", has been my conclusion for every round of pgbench-based testing 
I've done too.  In that case, the specific issue is that reads get 
blocked badly when checkpoint writes are doing heavier work; you can see 
the read I/O numbers reported by "vmstat 1" go completely to zero for a 
second or more when it happens.  That can happen with CFQ, too, but it 
consistently seems more likely to occur with deadline.

-- 
Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com  www.2ndQuadrant.com