Re: Undo header wait

Thanks for you reply. Actually the wait time is very small. Most of the time is zero.
Do not forget, I am a junir DBA, I thought as long as there is a wait, we should be alarmed. Now I understand, the wait time, not the wait event itself, is more important.

Thanks again

Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk> wrote in message news:1013967915.10296.0.nnrp-01.9e984b29_at_news.demon.co.uk...
>
> Can I just check my interpretation of the figures
> you have given here. Every hour, at about 22
> minutes past the hour, you get 10 to 20 waits
> for undo segment headers. If this is correct then . . .
>
> 1) How long are these waits ? Presumably a
> maximum of about 2/100 sec each - is the total
> really significant as the total wait time per hour ?
>
> 2) Have you have checked for other wait events
> to see of the total wait time elsewhere is perhaps
> more significant than 40/100 sec per hour ?
>
> 3) Is it possible that you have arranged your system
> so that checkpointing takes place at roughly hourly
> intervals - if so is it possible that these waits are the
> inevitable result of the fact that a process will have
> to wait occasionally whilst DBWR writes the rollback
> header to disc at a checkpoint ?
>
>
> --
> Jonathan Lewis
> http://www.jlcomp.demon.co.uk
>
> Now running 3-day intensive seminars
> http://www.jlcomp.demon.co.uk/seminar.html
>
> Host to The Co-Operative Oracle Users' FAQ
> http://www.jlcomp.demon.co.uk/faq/ind_faq.html
>
> Author of:
> Practical Oracle 8i: Building Efficient Databases
>
>
> Mike F wrote in message ...
> >After tried Howard Rogers's three suggestions, I still get the same
> >amount of undo header waits, and it all happens at the same time, one
> >hour difference.
> >
> >time event waits
> >2-15-2002 02:21 undo header 8
> >2-15-2002 02:22 undo header 6
> >2-15-2002 03:22 undo header 10
> >2-15-2002 03:24 undo header 9
> >2-15-2002 04:23 undo header 12
> >2-15-2002 04:26 undo header 8
> >2-15-2002 05:25 undo header 4
> >2-15-2002 05:26 undo header 6
> >..........
> >
> >I came across the same problem form Ixora web, but they did not
> >provide the solution, could somebody tell me what do they mean and how
> >should we deal with the problem?
> >
> >Thanks
> >
> >Q: Even I have over 12 rollback segments and the max concurrent
> >transaction is 4, I still get some undo header and undo block waits
> > from V$WAITSTAT. Is it possible to increase the FREELIST or FREELIST
> >GROUP for those rollback segments to alleviate this problem just like
> >what we do for normal tables or data segments?
> >
> >A:There are no such tuning options for rollback segments. This is
> >probably a problem with failing to retain undo blocks in cache for long
> >enough after they have been modified.
> >
> >How should we keep the undo blocks in cache for long enough? increase
> >db_block_buffer? But our hit ratio is very good. I am really confused
> >here.
> >
> >Thanks
> >
> >
> >
> >
> >
> >--
> >Sent by dbadba62 from hotmail piece from com
> >This is a spam protected message. Please answer with reference header.
> >Posted via http://www.usenet-replayer.com/cgi/content/new
>
>
Received on Sun Feb 17 2002 - 17:01:08 CST