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Oracle Tips by Burleson

Disk I/O Tuning

Chapter 3: SCSI Tuning Under AIX

You should also note that the original RS/6000 SCSI adapter does not support queuing.  It is inappropriate to attach a disk array device to such an adapter.

Controlling the Number of System pbufs in AIX

In AIX the Logical Volume Manager (LVM) uses a construct called a "pbuf" to control a pending I/O to disk.  In AIX Version 3, one pbuf is required for each page being read or written.  In systems that do large amounts of sequential I/O, this can result in depletion of the pool of pbufs.  The vmtune command can be used to increase the number of pbufs to compensate for this effect.

In AIX Version 4, only a single pbuf is used for each sequential I/O request, regardless of the number of pages involved.  This greatly decreases the probability of running out of pbufs and tuning pbufs in version 4 and is generally not advised.

In AIX Version 5, you no longer need to adjust this parameter.

The above text is an excerpt from:

Oracle Disk I/O Tuning
Disk IO Performance & Optimization for Oracle Databases
ISBN 0-9745993-4-4 

by Mike Ault

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