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FW: [Fwd: Response times with different multiblock read count]

From: Paul Drake <>
Date: Wed, 28 Mar 2001 11:14:46 -0800
Message-ID: <>


the difference here is :

select count(*) does not require the data in all of the columns. if you are going to be including columns that return a large amount of data, then the larger reads should help.

As its most likely that the OS read size is 64 KB, it makes sense that a local minimum of the response time function is found at that point.

Why don't you retry the query while bringing back all (or most) of the columns?



Paul Drake
Professional Software Systems

Hi All,

I am running a test query with different db_file_multiblock_read_count to test the overall throughput.

environment : oracle 8.1.6 on hp-ux v11 db_block_size : 16k
system is using LVM and file system is using buffered I/O and disk is mirrored but not stripped.

I was expecting that having larger multiblock count would result in better performance. But, when I was testing the query with smaller multiblock
count,the response time is better.

Here is the testing result.

Multiblock     time in Secs
----------     ------------
1           12.5   better
2              12.5   better
4              12.6   better
8              27     bad
16             27     bad
32             18.6   okay
64             19.0   okay

I don't understand why 8,16 are taking longer time. I did make sure that oracle is issuing proper multiblock read count with the help of multiblock_read_test.sql which is available in Steve Adams's site. Thank you steve for providing valuable information & scripts on your web site.

select statement that I am using is,
select /*+ full(t) noparallel(t) nocache(t) */ count(*) from &Table t;

I am clueless why it is behaving like this. Please pass your suggestions.
If you need more info like oracle/OS settings etc, please let me know.

When I was doing some reading on steve adam's site, he suggests the following.

Date: 29-Dec-2000 20:55
Nevertheless, it remains best to allow Oracle to use large multiblock reads, as long as the optimizer is not doing inappropriate full scans. The explanation relates to the system call and context switch overheads associated with I/O. It is cheaper in CPU usage terms to request a 256K I/O and allow the operating system to service it in 64K chunks, than to request 4 I/O's of 64K each. As is mentioned in the tip on Why Large Multiblock Reads, the increased
latency is not an issue because multiblock reads should not be used in cases where "first rows" response time is critical.

Thanks in advance.
Best Regards,

Please see the official ORACLE-L FAQ:

Author: Paul Drake

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