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ASM Inherently Performs Asynchronous I/O Regardless of filesystemio_options Parameter (Doc ID 751463.1)

Last updated on JANUARY 25, 2023

Applies to:

Oracle Database - Enterprise Edition - Version 10.2.0.1 and later
Oracle Database Cloud Schema Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Oracle Database Backup Service - Version N/A and later
Information in this document applies to any platform.
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Purpose

The purpose of this document is to explain the behaviour of I/O calls depending database parameters FILESYSTEMIO_OPTIONS and DISK_ASYNCH_IO when the Oracle RDBMS is located on ASM diskgroups.

Scope

FILESYSTEMIO_OPTIONS controls the behaviour of I/O options given by operating system but it has no affect when ASM is used. FILESYSTEMIO_OPTIONS enables asynchronous I/O (AIO) when the value is set to SETALL/ASYNC. However, ASM bypasses the filesystem layer in this case because ASM I/O is entirely controlled by DISK_ASYNCH_IO parameter. AIO needs to be enabled/disabled by setting disk_asynch_io to parameter values TRUE/FALSE.

In order to test this behaviour database writer process (DBW0) is traced with strace during a tablespace creation. An example strace of dbw0 process when AIO is used shows io_submit/io_getevents calls.

gettimeofday({1227275539, 15789}, NULL) = 0
times(NULL) = 532384532
io_submit(13086720, 2, {...}) = 2
gettimeofday({1227275539, 16181}, NULL) = 0
io_getevents(13086720, 1, 1024, {...}{600, 0}) = 1
times(NULL) = 532384532
io_getevents(13086720, 1, 1024, {...}{600, 0}) = 1



and the  strace of dbw0 process with AIO disabled shows pwrite64:

gettimeofday({1227274991, 462322}, NULL) = 0
times(NULL) = 532329776
pwrite64(22, "\36\242\0\0\3\0@\2\302\33\v\0\0\0\1\4VB\0\0\t\0\0\0\t\0\0\0\0\0\0\0"..., 8192, 6153068544) = 8192
times(NULL) = 532329776
times(NULL) = 532329776
pwrite64(24, "\36\242\0\0\3\0@\2\302\33\v\0\0\0\1\4VB\0\0\t\0\0\0\t\0\0\0\0\0\0\0"..., 8192, 6199205888) = 8192
times(NULL) = 532329776

 

Details

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In this Document
Purpose
Scope
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