RE: db_file_multiblock_read_count 10g default values
- From: "Allen, Brandon" <Brandon.Allen@xxxxxxxxxxx>
- To: Greg Rahn <greg@xxxxxxxxxxxxxxxxxx>, "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 5 Nov 2009 14:21:12 -0700
I've confirmed that it is based (at least partly, if not entirely) on the
db_cache_size as you can see below. I think the reason my numbers were varying
before is because I was using auto memory management, so the values I was
looking at for the db cache had changed since instance startup, but the
db_file_multiblock_read_count seems to only get adjusted at startup time.
If you plug all the values below into a spreadsheet, it looks like the formula
is roughly db_file_multiblock_read_count=db_cache_size/db_block_size*.003. To
put it another way, the maximum size of a multiblock read
(db_file_multiblock_read_count*db_block_size) is equal to .3% of the
db_cache_size, up to the max IO size of the OS, which is 1MB (or 128*8k) in my
case.
Regards,
Brandon
Consolidated spreadsheet:
db_cache_size dbfmbrc dbfmbrc*db_block_size(8k)
dbfmbrc*db_block_size/db_cache_size
------------- ------- -------------------------
--------------------------------------
117440512 41 335872 0.29%
167772160 59 483328 0.29%
218103808 76 622592 0.29%
318767104 112 917504 0.29%
419430400 128 1048576 0.25%
1073741824 128 1048576 0.10%
Raw data:
SYS@demo>alter system set db_cache_size=100m;
System altered.
SYS@demo>@parms
NAME VALUE
------------------------------ ----------
db_cache_size 117440512
db_file_multiblock_read_count 41
SYS@demo>alter system set db_cache_size=150m;
System altered.
SYS@demo>startup force;
SYS@demo>@parms
NAME VALUE
------------------------------ ----------
db_cache_size 167772160
db_file_multiblock_read_count 59
SYS@demo>alter system set db_cache_size=200m;
System altered.
SYS@demo>startup force;
SYS@demo>@parms
NAME VALUE
------------------------------ ----------
db_cache_size 218103808
db_file_multiblock_read_count 76
SYS@demo>alter system set db_cache_size=300m;
System altered.
SYS@demo>startup force;
SYS@demo>@parms
NAME VALUE
------------------------------ ----------
db_cache_size 318767104
db_file_multiblock_read_count 112
SYS@demo>alter system set db_cache_size=400m;
System altered.
SYS@demo>startup force
SYS@demo>@parms
NAME VALUE
------------------------------ ----------
db_cache_size 419430400
db_file_multiblock_read_count 128
SYS@demo>alter system set db_cache_size=1g;
System altered.
SYS@demo>startup force
SYS@demo>@parms
NAME VALUE
------------------------------ ----------
db_cache_size 1073741824
db_file_multiblock_read_count 128
-----Original Message-----
From: Greg Rahn [mailto:greg@xxxxxxxxxxxxxxxxxx]
I do know for sure it is related to parameters . . .
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