I had the opportunity to do more testing on another new server to see whether 
the kernel's I/O scheduling makes any difference.  Conclusion: On a 
battery-backed RAID 10 system, the kernel's I/O scheduling algorithm has no 
effect.  This makes sense, since a battery-backed cache will supercede any I/O 
rescheduling that the kernel tries to do.

Hardware:
Dell 2950
8 CPU (Intel 2GHz Xeon)
8 GB memory
Dell Perc 6i with battery-backed cache
RAID 10 of 8x 146GB SAS 10K 2.5" disks

Software:
Linux 2.6.24, 64-bit
   XFS file system
Postgres 8.3.0
max_connections = 1000 shared_buffers = 2000MB work_mem = 256MB max_fsm_pages = 1000000 max_fsm_relations = 5000 synchronous_commit = off wal_buffers = 256kB checkpoint_segments = 30 effective_cache_size = 4GB
Each test was run 5 times:
 drop database test
 create database test
 pgbench -i -s 20 -U test
 pgbench -c 10 -t 50000 -v -U test

The I/O scheduler was changed on-the-fly using (for example) "echo cfq 
>/sys/block/sda/queue/scheduler".

Autovacuum was turned off during the test.

Here are the results.  The numbers are those reported as "tps = xxxx (including connections 
establishing)" (which were almost identical to the "excluding..." tps number).

I/O Sched     AVG     Test1  Test2  Test3  Test4  Test5
---------    -----    -----  -----  -----  -----  -----
cfq           3355     3646   3207   3132   3204   3584
noop          3163     2901   3190   3293   3124   3308
deadline      3547     3923   3722   3351   3484   3254
anticipatory  3384     3453   3916   2944   3451   3156

As you can see, the averages are very close -- closer than the "noise" between 
runs.  As far as I can tell, there is no significant advantage, or even any significant 
difference, between the various I/O scheduler algorithms.

(It also reinforces what the pgbench man page says: Short runs aren't useful.  
Even these two-minute runs have a lot of variability.  Before I turned off 
AutoVacuum, the variability was more like 50% between runs.)

Craig

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