RE: db_file_multiblock_read_count causing full scans to takelonger?
- From: "Allen, Brandon" <Brandon.Allen@xxxxxxxxxxx>
- To: <kevin.lidh@xxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 19 Dec 2006 13:43:58 -0700
According to the docs, if you set it higher than your OS' max io size,
Oracle will automatically scale it down anyway:
http://download-west.oracle.com/docs/cd/B19306_01/server.102/b14237/init
params047.htm
"The maximum value is the operating system's maximum I/O size expressed
as Oracle blocks ((max I/O size)/DB_BLOCK_SIZE). If you set this
parameter to a value greater than the maximum, Oracle uses the maximum."
Just run a trace with WAITs and you will be able to see clearly how many
blocks it is in fact requesting in a single read, e.g.:
WAIT #6: nam='db file scattered read' ela= 42076 file#=14 block#=1035659
blocks=126 obj#=461389 tim=23362933581533
WAIT #6: nam='db file scattered read' ela= 16616 file#=15 block#=976523
blocks=126 obj#=461389 tim=23362947739977
WAIT #6: nam='db file scattered read' ela= 27336 file#=16 block#=954123
blocks=126 obj#=461389 tim=23362964451801
^^^^^^^^^^
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Kevin Lidh
My assumption is that if I set it higher, there is an attempt to make
the multiple I/O requests and assemble them into a single I/O response
and the overhead is the additional time.
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