Re: Oracle Performance Tuning - Tips & Techniques (Possible Error found) ?

Hi, kevin

As a rule of thumb, your attitude is correct. Since if you do the same work, then the "logical reads" will be the same.

According to the Oracle8 Server Tunning Manual, the new hit ratio is:

    hit ratio = 1 - (physical reads - ACH / (db block gets + consistent gets))

where ACH is the additional cache hits calculated from X$KCBRBH. Beware that ACH is not the increment of the DB_BLOCK_BUFFERS parameter.

 PS: There is no DB_DATA_BUFFERS parameter, it should be DB_BLOCK_BUFFERS.

Kevin A Lewis <KevinALewis_at_Hotmail.com> wrote in message news:AhVp3.4464$ld6.1442_at_newreader.ukcore.bt.net...
> I believe that in the genrally very good Preformance Tuning Tips &
> Techniques, I may have found a fault.
>
> The calculations published on pages 154 and 155 assume that Logical Gets
> (the sum of Consistent Gets and DB Block Gets) and Physical Reads are
> seperate counts. In other words the total reads are sum of both. As I
> understand it the Logical Gets are ALL the gets issued, with the Physical
> Reads being those that have to be satisfied from disk.
>
> This means that the calculations published to estimate the effect of
> increasing the DB_DATA_BUFFERS parameter subtract the hit adjustment from
> one side of the equation and adds it to the other, rather than just adjust
> the number of Physical Reads.
>
> e.g. on the Increase side
> NAME VALUE
> db block gets 10000
> consistent gets 90000 + 10000 (new logical reads added) = 100,000
> physical reads 20000 - 10000 (new physical reads lost) = 10,000
>
> hit ratio = 1 - Physical / Logical reads x 100%
> = (1 - (10000) / (10000 + 100000)) x 100 nb slightly
> adjusted
> = 91%
>
> I believe it should be more like
> NAME VALUE
> db block gets 10000
> consistent gets 90000
> physical reads 20000 - 10000 (new physical reads lost) = 10,000
>
> hit ratio = 1 - Physical / Logical reads x 100%
> = (1 - (10000) / (10000 + 90000)) x 100
> = 90%
>
> Please may I join the ranks of the fallible Oracle experts
>
> Regards
>
> --
> Kevin A Lewis (BOCM PAULS LTD - Animal Feed Manufacturer - Ipswich England)
> <KevinALewis_at_HotMail.com>
>
> The views expressed herein by the author of this document
> are not necessarily those of BOCM PAULS Ltd.
>
>
Received on Fri Aug 06 1999 - 00:33:58 CDT