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Re: Buffer Cache performance

From: Steve Adams <steve.adams_at_ixora.com.au>
Date: Mon, 01 Nov 1999 00:25:49 GMT
Message-ID: <381cd73e.19398844@news.eagles.bbs.net.au>


Hello Julio,

Table scans distort the cache hit rate. If done in parallel, they use direct I/O, which means that the physical reads are counted, but there are NO logical reads (consistent gets + db block gets). Understandably, this distorts the normal cache hit rate calculations.

Even serial table scans, which are performed through the cache by default, distort the cache hit rate significantly, because they get very few cache hits if any.

To get a better indication of whether there is room for improvement in the cache hit rate, you should attempt to calculate the cache hit rate exclusive of full table scans (and similar operations). There is a tip on my web site (first URL below) which explains how to do it. See "Calculating the Cache Hit and Miss Rates" on the "Tips" page.

Regards,
Steve Adams

http://www.ixora.com.au/

http://www.oreilly.com/catalog/orinternals/

http://www.christianity.com.au/



On Sun, 31 Oct 1999 14:02:27 -0000, "Julio Gosmaalho" <jgosmaalho_at_yahoo.com> wrote:

>Hi guys.
>
>In a Oracle 7.3 DB because of its slow performance i ran several scripts and
>I found that the buffer hit ratio is the worst value ( about 50%). All the
>other values ,redo, rollback segment, latch contention data dictionary, etc
>were ok (except library cache that needs a little adjustment - I think...).
>
>So I increased the init.ora parameter from 200 DB_BLOCK_BUFFERS from 200 to
>30.000, gradually - the memory of the NT Server allows that.
>
>RESULT - I only saw a benefit of 3% on the eficiency of the Buffer Cache
>!!!!!!!
>
>Is true that if the applications use full table scans, the buffer cache isnt
>used ?
>
>The strange thing is that in another site we have the same applications,
>same machine, with the same database and now the same parameters and the
>performance is good.
>
>Do you have any idea about it ??
>
>TIA
>
>CC
>
>
Received on Sun Oct 31 1999 - 18:25:49 CST

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