Re: slow i/o

The bgwriter parameters changed in 8.1

Try

bgwriter_lru_maxpages=0
bgwriter_lru_percent=0

to turn off bgwriter and see if there is any change.

-Jignesh

Junaili Lie wrote:
> 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
>
> On 8/29/06, *Jignesh K. Shah* <J(dot)K(dot)Shah(at)sun(dot)com
> <mailto:J(dot)K(dot)Shah(at)sun(dot)com>> wrote:
>
> Also to answer your real question:
>
> DTrace On Solaris 10:
>
> # dtrace -s /usr/demo/dtrace/whoio.d
>
> It will tell you the pids doing the io activity and on which devices.
> There are more scripts in that directory like iosnoop.d, iotime.d
> and others which also will give
> other details like file accessed, time it took for the io etc.
>
> Hope this helps.
>
> Regards,
> Jignesh
>
>
> Junaili Lie wrote:
> > Hi everyone,
> > We have a postgresql 8.1 installed on Solaris 10. It is running fine.
> > However, for the past couple days, we have seen the i/o reports
> > indicating that the i/o is busy most of the time. Before this, we
> only
> > saw i/o being busy occasionally (very rare). So far, there has
> been no
> > performance complaints by customers, and the slow query reports
> doesn't
> > indicate anything out of the ordinary.
> > There's no code changes on the applications layer and no database
> > configuration changes.
> > I am wondering if there's a tool out there on Solaris to tell which
> > process is doing most of the i/o activity?
> > Thank you in advance.
> >
> > J
> >
>
>