Re: CR Block copies, always 6 in

From: K Gopalakrishnan <>
Date: Mon, 24 Mar 2008 21:07:08 -0700
Message-ID: <>


You are right about the BBW timeout of 1sec. But IIRC this was little different from regular BBW and I will have to go back to my notes to confirm the 10cs behavior.

Let us not blame the RAC or lightwork rule for limiting number CR buffers in the buffer cache. In fact there is no hard limit for number of cr copies in RAC or single instance. This is just a soft limit - means this limit is enforced only when there is a memory pressure in the buffer cache. During that time, upto 6 copies of the CR buffers are allowed to be part of the hot part MRU list and the remaining CR buffers are linked to the Auxiliary list which are ready for expiry. If your buffer cache is sufficiently large and there is no memory pressure, we don't need to enforce the number of cr copies limitation. This is one of the reasons for not able to simulate this behavior in the simple lab tests.

> It's a bit vague that i can't 100% understand what the author is trying to
> say.
> Seems that he is referring buffer busy waits wait event but the timeout of
> buffer busy wait is 1s(100cs) not 10cs.
> Asking it to the author seems to be good solution.
> Jonathan Lewis has stated a quite probable answers to this situtation.
> My additional guesswork is that Oracle generates more CR blocks than the
> number limited by _db_block_max_cr_dba
> when light work rule is applied on RAC system.
> But I don't have access to RAC at the moment that can't verify it.
> Dion Cho

Best Regards,
K Gopalakrishnan
Co-Author: Oracle Wait Interface, Oracle Press 2004

Author: Oracle Database 10g RAC Handbook, Oracle Press 2006
Received on Mon Mar 24 2008 - 23:07:08 CDT

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