RE: db_file_multiblock_read_count 10g default values

  • From: "Christian Antognini" <Christian.Antognini@xxxxxxxxxxxx>
  • To: "Allen, Brandon" <Brandon.Allen@xxxxxxxxxxx>
  • Date: Fri, 6 Nov 2009 10:19:34 +0100

Hi Brandon

> After a little more fiddling, it looks like the formula is
> calculated at instance startup as:
>
> db_file_multiblock_read_count = db_cache_size/processes/9600,
> with a max of .28% of the db_cache_size
>
> I'm not sure where that constant 9600 comes from, but I'd guess
> it's based on db_block_size (8192 in my case) plus some other
overhead.

I don't know if it's helpful... Anyway, some years ago I investigated
this issue. My conclusion, which was also published in my book (page
178), was the following:

dfmbrc = min(1048576/db_block_size,
db_cache_size/sessions/db_block_size)


BTW, the dependency between db_cache_size and dfmbrc is, IMO, really
bad... As a result, I never use this "auto-tune" feature. For this
reason, back in 2005 I opened an SR about it. After some time the
related bug was recognized to be a code bug. Few month ago (after almost
4 years of inactivity!) they changed the "status" to "FixBy->12". So, it
might be that, at last, they'll solve the problem in the next major
release...


HTH
Chris Antognini

Troubleshooting Oracle Performance, Apress 2008
http://top.antognini.ch

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