Re: slow i/o - Mailing list pgsql-performance

Hi Jignesh,
Thank you for my reply.
I have the setting just like what you described:

wal_sync_method = fsync
wal_buffers = 128
checkpoint_segments = 128
bgwriter_all_percent = 0
bgwriter_maxpages = 0

I ran the dtrace script and found the following:
During the i/o busy time, there are postgres processes that has very high BYTES count. During that non i/o busy time, this same process doesn't do a lot of i/o activity. I checked the pg_stat_activity but couldn't found this process. Doing ps revealed that this process is started at the same time since the postgres started, which leads me to believe that it maybe background writer or some other internal process. This process are not autovacuum because it doesn't disappear when I  tried turning autovacuum off.
Except for the ones mentioned above, I didn't modify the other background setting:
MONSOON=# show bgwriter_delay ;
 bgwriter_delay
----------------
 200
(1 row)

MONSOON=# show bgwriter_lru_maxpages ;  bgwriter_lru_maxpages
-----------------------
 5
(1 row)

MONSOON=# show bgwriter_lru_percent ;
 bgwriter_lru_percent
----------------------
 1
(1 row)

This i/o spike only happens at minute 1 and minute 6 (ie. 10.51, 10.56) . If I do select * from pg_stat_activity during this time, I will see a lot of write queries waiting to be processed. After a few seconds, everything seems to be gone. All writes that are not happening at the time of this i/o jump are being processed very fast, thus do not show on pg_stat_activity.

Thanks in advance for the reply,
Best,

J