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Re: Rollback writes >highwater mark size

From: Dave Hau <davehau_nospam_123_at_nospam_netscape.net>
Date: Wed, 02 Jul 2003 20:36:03 GMT
Message-ID: <3F034232.4050909@nospam_netscape.net>


michael ngong wrote:
> Database Enterprise Edition 8.1.7.0.0
> OS 8.1.7 Hp Unix 11.0
>
> It is kind of interesting that the rollback stats below almost
> consistently show the HWMSIZE is lower than the writes.

HWMSIZE < WRITES is normal because the rollback segment is a circular buffer of extents. It wraps around instead of extending indefinitely. Therefore, eventually the number of bytes written will cause the rollback segment to wrap around, i.e. WRITES > HWMSIZE.

> I also expected some of the rollback segments to have optimal size as
> a result of shrinking.

To evaluate this, you need to compare OPTSIZE to RSSIZE, not HWNSIZE which by definition is a *high water mark* of rollback segment size and won't shrink. The goal of setting the optimal size is to get OPTSIZE to be close to AVEACTIVE and to get SHRINKS to a low value.

Cheers,
Dave

>
> Configuration of initial and next..Initial extent 4194304 next_extent
> 4194304
> Just in case your documentation is far away below are the definitions
> from the docs for a reminder.
> WRITES
> The number of bytes of entries written to the rollback segment.
>
>
> OPTSIZE
> The value of the optimal parameter for the rollback segment.
>
> HWMSIZE
> The highest value (high water mark), in bytes, of the rollback
> segment size reached during usage.
>
> SHRINKS
> The number of shrinks that the rollback segment has had to perform in
> order to stay at the optimal size.
>
> WRAPS
> The number of times a rollback segment entry has wrapped from one
> extent to another.
>
> EXTENDS
> The number of times that the rollback segment had to acquire a new
> extent.
>
>
>
> SQL> select writes,optsize,hwmsize,shrinks,wraps from v$rollstat;
>
> WRITES OPTSIZE HWMSIZE SHRINKS WRAPS
> ---------- ---------- ---------- ---------- ----------
> 2580 5251072 0 0
> 121459630 16777216 109682688 3 33
> 52427030 16777216 16867328 0 15
> 39317908 16777216 16867328 0 11
> 140816400 16777216 113901568 3 39
> 167348988 33554432 117547008 1 23
>
>
> Appreciater your input as usual
>
> Michael Tubuo Ngong
Received on Wed Jul 02 2003 - 15:36:03 CDT

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