FAQ

On Thu, Sep 4, 2008 at 1:49 PM, Lou Avrami wrote:
I have inherited several Oracle 9.2.0.8 data warehouses on HP-UX 11.11 servers, each 2+ TBs. All of them are experiencing various performance issues.
What metrics do you have that points to I/O being an problem?
The db_multiblock_read_count (MBRC) on these databases are explicitly set to the value of 8.
This might be a bit small for a warehouse, but it completely depends
on the query workload. Do the majority of the queries use index
access or table scans? Adjusting the MBRC will effect the costing of
*all* plans, but it will only benefit the I/O sizes for those doing
partition/index fast full/table scans. Be careful what you optimize
for...if your storage and I/O channels can not handle more I/O from
more FTS then there will likely be little benefit in adjusting it
larger. In fact, if your I/O is bottlenecked, it may get worse.
To try to get a better idea of what we might increase the MBRC to, I wanted to figure out the values for the formula:
[stripe width] x [stripe size] / [db_block_size ]
This looks like a great equation but I don't think it is is what you
want. Generally it is better to have the largest database I/O match
the stripe depth/size (how much of the stripe is on a single physical
disk) so that a single large (say 1MB) I/O is serviced by exactly one
physical disk.

I would recommend seeing the Oracle docs:
http://download.oracle.com/docs/cd/B28359_01/server.111/b28274/iodesign.htm#i28412

The other thing to consider is all the different layers that
change/chop I/O size. This includes kernel, I/O driver, storage
array, raid controller, etc. The size I/O that you think you are
sending may not be what the physical disk actually gets. This is only
visible with storage array tools.
I realize that perhaps the best way to determine the optimal MBRC value with these file systesm would be to use the dd command and write a large datafile with different block size options to the filesystems.
Actually the best way to determine it is to run some FTS and alter
MBRC at the session level comparing the elapsed times. Just make sure
that you are doing 100% physical I/O (no blocks from buffer cache or
filesystem cache). Then you will see if it is worth your time. My
guess is probably not.

I would recommend your spend your time looking at execution plans and
determining if you have optimal partitioning. The fastest I/O is the
one that never takes place (e.g. partition elimination). These will
likely give you orders of magnitude gains vs. a few percentage points
of gains tweaking MBRC. Go after the big fish!

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postedSep 4, '08 at 8:49p
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