Linux I/O tuning: CFQ vs. deadline

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