Re: iostat and Rollback Segments

Ethan Post wrote:
>
> Running some testing thats simple purpose is to commit as frequently as
> possible and generate redo. Watching iostat I see redo showing 20%
> utilization. The volume containing rollback segment maxes out at around .8
> MB per second and shows 100% utilization. I have moved the rollback
> segments around and tried different sizes. What I see is the same volume
> jump to 15 MB per second at times (i.e. copying an archive log), this can be
> sustained for 10-20 seconds or so. How come the very same volumes max out
> at .8 MB per second for rollback segments?
>
> Also in playing around with rollback segment sizes I noticed dramatic
> differences in amount writen to the rollback segment in the writes column of
> v$rollstat.
>
> The test runs at almost the exact same rate with all iterations of rollback
> segment sizes and placement. Disk always hits 100% on rollback segment
> volume but I see no waits on any rollback segments. Writes increases
> greatly based on RBS size but time to complete is always the same.
>
> hum...?
>
> Thanks,
> Ethan Post

Are you sure you are actually peaking the rollback segs physically - Oracle is going to try keep them in the cache and avoid the disk altogether if it can...

A manual shrink will force physical IO - maybe setup a large shrink and see what the disks do.

hth
connor

-- 
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Connor McDonald

http://www.oracledba.co.uk

"Some days you're the pigeon, some days you're the statue..."