RE: db_file_multiblock_read_count causing full scans to takelonger?
- To: "Allen, Brandon" <Brandon.Allen@xxxxxxxxxxx>
- Date: Tue, 19 Dec 2006 21:52:06 +0100
Brandon
> Am I missing something, or isn't it best to just leave this parameter
> unset (default) in 10.2, in which case Oracle will automatically
adjust
> it to the max io size of your OS?
Mhmm... I don't like that for the simplest reason that the maximum is
not the best value for performance! As the OP noticed...
> Along with computing system stats on a typical workload so you have an
> accurate mbrc, you should get the best of both worlds - accurate
costing
> and optimized sequential scans, right?
In some situations, as I just wrote, no.
> I have it unset on my 10.2 AIX 5.3 system and it has defaulted to 128
> with an 8k block size (1MB).
Notice that there are some limits... According to my tests the value of
db_file_multiblock_read_count is calculated with the following formula:
min( DbCacheSize/(Sessions*DbBlockSize) , 1048576/DbBlockSize )
Regards,
Chris
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