Re: iostat and Rollback Segments

Ethan Post <Blah_at_Blah.com> 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?

Seeks.

If the disk head has to move then that's several ms that you can't do I/O on that disk. As you see, when you're doing nearly no seeks (copying a single large file), the disk manages 15MB/s. A fast SCSI disk will seek in a nominal 4ms. If you're doing 200 seeks/second, the disk will be spending 80% of its time moving the head, and only 20% doing I/O.

Redo generation is fairly nice to disks, there's lots of annoying small writes, but it's all fairly linear. Rollback segments are written by DBWR as and when it feels like, and will be far more scattered.

I prefer to look at sar for disk stats. My rule of thumb for max performance is more than 10ms service time is poor, for redo keep it below 6ms.

A good write-cache on the disk array can do wonders for the service time, just make sure it's battery backed.  

-- 
Andrew Mobbs - http://www.chiark.greenend.org.uk/~andrewm/