Re: disk_asynch_io and db_writer_processes on RAW DBFs

On 02/19/2006 10:56:11 PM, Deepak Sharma wrote:
> OS: Sun Solaris 2.8
> Database: Oracle 10.1.0.4
> CPU_COUNT: 32
>
> On a database using RAW datafiles, what values
> should be specified for "db_writer_processes"
> and/or "filesystemio_options", assuming that
> "disk_asynch_io=TRUE" (also async i/o is available at
> OS-level) ?

There is Oracle's on-line support page called Metalink. Its URL is http://metalink.oracle.com. My understanding is that this list is not a plug-in replacement for metalink. Having said that and given your repeatedly demonstrated unwillingness to do your own homework, let me answer the question. There is a metalink note 47328.1 with the following title: Reference Note for Init.Ora Parameter "DISK_ASYNCH_IO". To some people, me included, it would look like a clue for searching the answers. Here is what it says:

"DISK_ASYNCH_IO controls whether I/O to datafiles, control files, and logfiles is asynchronous (that is, whether parallel server processes can overlap I/O requests with CPU processing during table scans). If your platform supports asynchronous I/O to disk, Oracle Corporation recommends that you leave this parameter set to its default value. However, if the asynchronous I/O implementation is not stable, you can set this parameter to false to disable asynchronous I/O. If your platform does not support asynchronous I/O to disk, this parameter has no effect."

and then:

  "If DISK_ASYNCH_IO is set to FALSE, then typically either   <Parameter:DBWR_IO_SLAVES> or <Parameter:DB_WRITER_PROCESSES> should   be set unless the system is low throughput."

My understanding of that is that if you have asynchronous I/O, it will be used automatically. DB_WRITER_PROCESSES should be increased only if you experience extensive waits on buffer LRU latch. IF you're using dynamic SGA, this is automatically computed for you, as per Metalink note 148495.1 Finally, the filesystemio_options do not matter for raw devices as raw devices are not filesystem. You automatically have asynchronous and direct I/O on raw devices.

I have the following questions:

1. What kind of applications will be running on this system? If it's a data warehouse, contrary to the popular belief, raw data files are not particularly beneficial.
2. What are you tuning? Why would you want to modify these parameters? Do you have performance problems?
3. Are you sure that you know what are you doing? DBA administration is more then just tweaking init.ora parameters.

> __________________________________________________
> Do You Yahoo!?

Occasionally. Do you Metalink?

-- 
Mladen Gogala
http://www.mgogala.com

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