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Re: Undo header wait

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Sun, 17 Feb 2002 17:45:00 -0000
Message-ID: <1013967915.10296.0.nnrp-01.9e984b29@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 ?
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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
>
>
>
>
>
>--
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Received on Sun Feb 17 2002 - 11:45:00 CST

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