Re: How to confirm 'direct io' on linux?

From: Mladen Gogala <gogala_at_sbcglobal.net>
Date: Thu, 25 Aug 2005 00:21:35 GMT
Message-Id: <pan.2005.08.25.00.21.31.934001@sbcglobal.net>

On Tue, 23 Aug 2005 20:21:23 +0000, NetComrade wrote:

> We are currently testing running Oracle 9.2.0.6 on Redhat 4.0 AMD x64.
> We are using Veritas File System, and ODM..
> We have linked odm.so files into $oracle-home/lib
> On solaris, I believe OS calls should've been ODM, or at least kaio
> (when using truss), what should we be seeing on linux when using
> strace?

open(....,O_DIRECT) is what you should see. Direct I/O on Linux is available on Ext3 and JFS which are free. I haven't tested VxFS but I can not imagine that it would not perform direct I/O when instructed to do so. Here is how you check: $ svrmgrl

SQL*Plus: Release 10.2.0.1.0 - Production on Wed Aug 24 20:13:24 2005

Connected to an idle instance.

SQL> startup mount
ORACLE instance started.

Total System Global Area 243269632 bytes

Fixed Size                  1218748 bytes
Variable Size              71305028 bytes
Database Buffers          167772160 bytes
Redo Buffers                2973696 bytes

Database mounted.
SQL> !ps -ef|grep dbw
oracle 4681 1 0 20:13 ? 00:00:00 ora_dbw0_10g oracle 4703 4672 0 20:13 pts/0 00:00:00 /bin/ksh -c ps -ef|grep dbw oracle 4707 4703 0 20:13 pts/0 00:00:00 grep dbw

SQL> Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production With the Partitioning, OLAP and Data Mining Scoring Engine options $
$ strace -o /tmp/dbwr.out -p 4681 & (Process 4681 is the database writer) [1] 4708
$ Process 4681 attached - interrupt to quit

$ svrmgrl

SQL*Plus: Release 10.2.0.1.0 - Production on Wed Aug 24 20:15:33 2005

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production With the Partitioning, OLAP and Data Mining Scoring Engine options

SQL> alter database open;

Database altered.

SQL> select count(*) from scott.emp;

COUNT(*)

SQL>
$ grep O_DIRECT /tmp/dbwr.out
open("/oradata/10g/oracle/datafile/10G/datafile/o1_mf_system_1j2cfzfv_.dbf", O_RDONLY|O_DIRECT|O_LARGEFILE) = 19 open("/oradata/10g/oracle/datafile/10G/datafile/o1_mf_system_1j2cfzfv_.dbf", O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE) = 19

fcntl64(19, F_GETFL)                    = 0xd002 (flags O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE)
open("/oradata/10g/oracle/datafile/10G/datafile/o1_mf_undotbs1_1j2cfzm4_.dbf", O_RDONLY|O_DIRECT|O_LARGEFILE) = 20
open("/oradata/10g/oracle/datafile/10G/datafile/o1_mf_undotbs1_1j2cfzm4_.dbf", O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE) = 20
fcntl64(20, F_GETFL)                    = 0xd002 (flags O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE)

fcntl64(21, F_GETFL)                    = 0xd002 (flags O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE)

fcntl64(22, F_GETFL)                    = 0xd002 (flags O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE)
open("/oradata/10g/oracle/datafile/10G/datafile/o1_mf_perfstat_1j2f56l5_.dbf", O_RDONLY|O_DIRECT|O_LARGEFILE) = 23
open("/oradata/10g/oracle/datafile/10G/datafile/o1_mf_perfstat_1j2f56l5_.dbf", O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE) = 23
fcntl64(23, F_GETFL)                    = 0xd002 (flags O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE)

fcntl64(24, F_GETFL)                    = 0xd002 (flags O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE)

fcntl64(25, F_GETFL)                    = 0xd002 (flags O_RDWR|O_SYNC|O_DIRECT|O_LARGEFILE)

By definition, every I/O to the handle opened by O_DIRECT bypasses system buffering and goes directly to the user buffers instead. At the end, I humbly confess that I am lazy and therefore using OMF (Oracle Managed Files).

-- 
http://www.mgogala.com

Received on Wed Aug 24 2005 - 19:21:35 CDT