Re: Improvement of checkpoint IO scheduler for stable transaction responses

I create fsync v2 patch. There's not much time, so I try to focus fsync patch in
this commit festa as adviced by Heikki. And I'm sorry that it is not good that
diverging from main discussion in this commit festa... Of course, I continue to
try another improvement.

* Changes
- Add ckpt_flag in mdsync() etc with reference by Heikki's patch. It will be
more controllable mdsync() in checkpoint.
- Too long sleep in fsync phase is not good for checkpoint schedule. So I set
limited sleep time which is always less than 10 seconds(MAX_FSYNC_SLEEP).
I think that 10 seconds sleep time is a suitable value in various situations.
And I also considered limited sleep time by checkpoint progress,
however, I thought md.c should be simple and remain robust. So I have remained
simple.
- Maximum checkpoint_fsync_sleep_ratio in guc.c is changed 1 to 2. Because I
set limited sleep time 10 secounds. We can more flexibly change it and be more
safety.

And I considered abbreviation of parameters in my fsync patch.
* checkpoint_fsync_delay_threshold
In general, I think that it is suitable about 1 second in various environments.
If we want to adjust sleep time in fsync phase, we can change
checkpoint_fsync_sleep_ratio.

* checkpoint_fsync_sleep_ratio
I don't want to omit this parameter, because it can only regulate sleep time
in fsync phase and checkpoint time.

* Benchmark Result(DBT-2)
| NOTPM Average 90%tile Maximum
------------------------+----------------------------------------
original_0.7 (baseline) | 3610.42 4.556 10.9180 23.1326
fsync v1 | 3685.51 4.036 9.2017 17.5594
fsync v2 | 3748.80 3.562 8.1871 17.5101

I'm not sure about this result. Fsync v2 patch was too good. Of cource I didn't
do anything in executing benchmark.
Please see checkpoint_time.txt which is written detail checkpoint in each
checkpoint. Fsync v2 patch seems to be short in each checkpoint time.

* Benchmark Setting
[postgresql.conf]
archive_mode = on
archive_command = '/bin/cp %p /pgdata/pgarch/arc_dbt2/%f'
synchronous_commit = on
max_connections = 300
shared_buffers = 2458MB
work_mem = 1MB
fsync = on
wal_sync_method = fdatasync
full_page_writes = on
checkpoint_segments = 300
checkpoint_timeout = 15min
checkpoint_completion_target = 0.7
segsize=1GB(default)

[patched postgresql.conf (add)]
checkpointer_fsync_delay_ratio = 1
checkpointer_fsync_delay_threshold = 1000ms

[DBT-2 driver settings]
SESSION:250
WH:340
TPW:10
PRETEST_DURATION: 1800
TEST_DURATION: 1800

* Test Server
Server: HP Proliant DL360 G7
CPU: Xeon E5640 2.66GHz (1P/4C)
Memory: 18GB(PC3-10600R-9)
Disk: 146GB(15k)*4 RAID1+0
RAID controller: P410i/256MB
(Add) Set off energy efficient function in BIOS and OS.

Best regards,
--
Mitsumasa KONDO
NTT Open Sorce Software Center