Re: slow i/o

I have tried this to no avail.
I have also tried changing the bg_writer_delay parameter to 10. The spike in
i/o still occurs although not in a consistent basis and it is only happening
for a few seconds.

On 8/30/06, Jignesh K. Shah <J(dot)K(dot)Shah(at)sun(dot)com> wrote:
>
> 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
> > >
> >
> >
>