Re: LGWR using lots of CPU time, low CPU usage

> >And "old school" is still right about not putting RedoLogs onto RAID5.

> From what I'm being told, this is not your father's RAID5. This is what

>they tell me:

> The CPU hands the IO to the disk controller and rather than do the
> physical disk IO while the process waits, the disk controller caches
> it to local memory and says done. Therefore, effectively there is no
> wait for IO and it doesn't matter if we are RAID 5 or RAID 0+1,
> the system is NOT waiting for the IO. He said the only time there might
> be a delay is during the cache's battery refresh times. I checked your
> dates and it was not occurring during those times. Also, if you look
> at the iostat statistics under the 'wait' and '%w' headers you will
> see all zeros.

Debi,

That is true, up to a point.

Think of the cache as a water tank. You have a garden hose filling up the tank. You can keep increasing the water pressure for a while.

But the outlet at the other end of the tank has a fixed capacity. It flows 10 GPM, and no more.

What happens when you increase the flow at the intake to 20 GPM? The tank fills up.

When the tank fills up, your intake flow will need to decrease, because you can only flow 10 GPM at the outlet.

Now, think of the outlet as writing to disk, the RAID5 cache is the water tank, and your database is the inlet that wants to run at 20 GPM.

If your database activity will never be intensive enough to stress the cache like this, no problem. But 'never' is a very long time.

If any of this sound familiar, Cary Millsap posted a very similar explanation a few weeks ago.

Plagierism is the sincerest form of flattery. :)

Jared

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