Re: Cache Hit Ratio from system views

From: Niall Litchfield <niall.litchfield_at_dial.pipex.com>
Date: Wed, 08 Aug 2007 21:11:14 +0100
Message-ID: <rKmdnQiZ1NVLgifbnZ2dnUVZ8seinZ2d@pipex.net>

sybrandb_at_hccnet.nl wrote:
> On Wed, 08 Aug 2007 12:02:46 -0700, Teresa Masino
> <teresa.masino_at_peninsula.org> wrote:
>

>> Alone, yea, but they can sometimes be an indicator that an instance
>> needs attention.

> Do you really think so? Yesterday I had a customer spouting flames
> because of a RAC database coming down to a crawl due to buffer busy
> waits in conjunction with both instances updating the same table over
> and over again.
> Nothing to be seen from the BCHR.
>
> But then of course BCHR is a MIPS (Meaningless Indicator of the
> Performance of the System).
> Only good old Don Burleson makes people erroneously believe they can
> tune their database by throwing memory at the problem to crank up the
> BCHR.
> Savy professionals (a favorite phrase of Don) know better.
>

enough of you will recognise where I stand on ratios. Indeed I believe that I presented on the effectiveness or in fact otherwise of the BCHR on its own nearly 5 years ago.

I have some sympathy withe Teresa's sentiment above though. Imagine a system that has had a truly horrible burleson concerned hopelessness ratio of 75% since 2002. Today the BCHR is 25% (or indeed 98%). I venture to suggest that it's a change worth determining the reasons for, either way. Ratios don't mean anything much, abrupt changes in them often do.

-- 
Niall Litchfield
Oracle DBA
http://www.orawin.info/services

Received on Wed Aug 08 2007 - 15:11:14 CDT