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RE: RE: Re[2]: db_file_multiblock_read_count parameter

From: Steve Adams <>
Date: Sat, 15 Jul 2000 06:14:51 +1000
Message-Id: <>

Hi Dick,

OK, that makes better sense. It was the "in any one time slice" bit that was missing from your previous mail. This person was clearly speaking in the context of buffered file system I/O, and in that case he/she is right. HP-UX (and buffered Unix file systems in general) will only read one file system buffer (typically 8K) from disk at a time, and unless you are reading from a cached disk array, you would be hard pressed to read more than 32K in a timeslice. However, there is no hard coded 32K limit anywhere.

Steve Adams

-----Original Message-----
From: []
Sent:	Friday, July 14, 2000 11:30 PM
To:	Steve Adams; ''
Subject:	Re:RE: Re[2]: db_file_multiblock_read_count parameter


    Some time ago I changed a DB from 8 to 800 on the DB_FILE_MULTIBLOCK_READ_COUNT, as a suggested performance improvement, with disastrous results. I asked OWWS what was going on & they referred me to HP.
To get things back under control I returned the DB's DB_FILE_MULTIBLOCK_READ_COUNT to 8 which was a very dramatic improvement & put
the matter on the list of items to discuss with our HPCE on his next pending
visit. Now I wish I could remember the guys name because he earned a great deal
of respect from me on his capabilities with HP-UX, all I can remember is that
his first name was Ben. He was later, to my great consternation, transferred to
Atlanta by HP & I understand was one of the authors of HP-UX 11. Anyhow, he was
as much puzzled as I was & took the question back to the office with him.  About
a month later I got an e-mail from an individual at HP's Atlanta office which
contained a page from some manual that I have never been able to get hold of
that basically said that "irrespective of other parameters, the OS will not return more than 32K of data from disk to any one process in any one time slice". His recommendation was to set DB_FILE_MULTIBLOCK_READ_COUNT * DB_BLOCK_SIZE <= 32K. I followed his recommendation with remarkable results,
many end users wanted to know if we had purchased a new server. And have stuck
to it to this day WITHOUT getting burned or even scorched. I did try a larger
value recently & although it did not misbehave as before, I was disappointed by
the results. At any rate, I'm NOT an SA & do not profess to be a UNIX Guru,
more a jack of all OS's and a master of NONE. But your question did spawn a
second look out on Metalink. May all find the attached helpful.

Dick Goulet

    Unix Installation/OS: RDBMS Technical Forum   Displayed below are the messages of the selected thread.

  Thread Status: Closed

  From: Robert Hernandez 14-Jun-00 21:19   Subject: max db_block_size*db_file_multiblock_read_count for different OS

  RDBMS Version: Oracle 8.x.x
  Operating System and Version: HPUX-11.0,SUN 7.0/8.0,AIX 4.3   Error Number (if applicable):
  Product (i.e. SQL*Loader, Import, etc.):   Product Version: 8.x.x

  max db_block_size*db_file_multiblock_read_count for different OS

  My company is just finishing a new warehouse product running on Oracle 8.x.x
Enterprise Edition and HPUX 11.0. We are thinking to port it now to different OS
platform. I looking for the following information to help me recommend we OS
platform we should support next.

  I understand the max db_block_size*db_file_multiblock_read_count for   HP-UX 10.20 is 64K
  HP-UX 11.0 is 256K

  but what about AIX 4.3

  SUN 2.6
  SUN 7.0
  SUN 8.0

  NT 4.0
  NT 2000
  And what is the max DB_BLOCK_SIZE for each OS running Oracle 8.1.6 EE?

  From: Oracle, Anand Viswanathan 15-Jun-00 15:05   Subject: Re : max db_block_size*db_file_multiblock_read_count for different OS


  db_block_size * db_file_multiblock_read_count is controlled by   max io that os can do.

  maxio is controlled by the kernel parameters maxphys for UFS and   vol_maxio for veritas filesystem on solaris.

  You have got to know the max value the os can support from the os   vendors for your versions.

  Irrespective of this ,we have got a kernel constant SSTIOMAX which   controls the maxio that particular rdbms version can support.

  So if both os side values and SSTIOMAX from the rdbms version   supports having a io size as specified by your db_block_size *   db_file_multiblock_read_count ,then your setting of db_file_multiblock_read_count is taken ,otherwise it defaults to lower value.

  On Oracle 8 it supports 512K,so if your os supports for this   parameter maxphys to be equal to 512K,then for block size of 8K   you can have a db_file_multiblock_read_count of 64.

  Similarly in other platforms you have to ensure the os kernel   parameters which controls the io size and to find out the max value   that can be supported for that os from the os vendor ,then if   you are using Oracle 8 then you can go upto iosize of 512K.



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____________________Reply Separator____________________
Subject: RE: Re[2]: db_file_multiblock_read_count parameter Author: Steve Adams <> Date: 7/14/00 9:53 AM

Hi Dick,

What makes you think that HP-UX has a limit of 32K per read? On buffered file systems all reads match the file system buffer size (normally
but on raw or direct I/O you can go all the way up to MAXPHYS which has long been 256K and is now 1M.

Steve Adams

-----Original Message-----
From: [] Sent: Friday, July 14, 2000 1:09 AM

To:     Multiple recipients of list ORACLE-L
Subject:        Re[2]: db_file_multiblock_read_count parameter

One word of caution. If you raise this parameter too far the operating system
may well ignore you. In HP-UX (the OS we use) the limit is 32K of data per read
therefore taking the db_block_size of 8k means that any value of DB_FILE_MULTI_BLOCK_READ_COUNT > 4 gets ignored.

Dick Goulet

____________________Reply Separator____________________
Author: Gaja Krishna Vaidyanatha <>
Date:       7/12/00 3:36 PM


It is a dynamic parameter that can be modified using an "alter system" or an "alter session" as the case may be.



Gaja Krishna Vaidyanatha
Director, I-O Management Products
Quest Software Inc.

"Opinions and views expressed are my own and not of Quest"

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Received on Fri Jul 14 2000 - 15:14:51 CDT

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