Re: Raw Devices: Increased Performance?

Jonathan Lewis <ora_mail_at_jlcomp.demon.co.uk> wrote:

>Joel Garry wrote:
>>
>> An unintended side effect is, poorly optimized apps that force too
>> many full-table scans will do better with raw devs...
>>
 

>I would have said that there is an opposing argument to that.
 

>If you do tablescans in Oracle, the data blocks are not kept in the
>SGA, so a repeated tablescan does the same set of disk reads agsin.

Simply not true. You can prove it easily yourself when doing a join between two tables (which of course must result in full table scans) and watching your system.

CPU utilization will be at 100%, without any physical I/O activity.

>With a file system, the disk reads may be satisfied from the
>file system buffer. With raw devices they will be physical
>reads. Result: a bad app collapses when moved to raw disk.

If you simply switch to raw devices, this might be true.

Consequently, when you switch to raw, you should allocate your (no unused) UNIX-cache to the Oracle SGA (using db_buffers). This way, Oracle will find the data in its own cache....

>---
>Jonathan Lewis
>ora_mail_at_jlcomp.demon.co.uk

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