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ASM using ASMLib and Raw Devices

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Introduction

Automatic Storage Management (ASM) simplifies administration of Oracle related files by allowing the administrator to reference disk groups rather than individual disks and files, which ASM manages internally. On Linux, ASM is capable of referencing disks as raw devices or by using the ASMLib software. This article presents the setup details for using either raw devices or ASMLib, as well as the procedures for converting between both methods.

The article assumes the operating system installation is complete, along with an Oracle software installation. The ASM instance shares the Oracle home with the database instance. If you plan on running multiple database instances on the server the ASM instance should be installed in a separate Oracle home.

Note: When running Oracle 10g Release 2 on RHEL 4 you should consider reading this article: Using Block Devices for Oracle 10g Release 2 in RHEL 4

Partition the Disks

Both ASMLib and raw devices require the candidate disks to be partitioned before they can be accessed. In this example, three 10Gig VMware virtual disks are to be used for the ASM storage. The following text shows the "/dev/sdb" disk being partitioned.

# ls sd*
sda  sda1  sda2  sdb  sdc  sdd
# fdisk /dev/sdb
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.


The number of cylinders for this disk is set to 1305.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)
Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-1305, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-1305, default 1305):
Using default value 1305

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
#

The remaining disks ("/dev/sdc" and "/dev/sdd") must be partitioned in the same way.

ASMLib Installation

This step is only necessary if you want to use ASMLib to access the ASM disks.

Determine your kernel version using the following command as the root user.

# uname -r
2.6.9-34.ELsmp
#

Download the ASMLib software from the OTN website, making sure you pick the version that matches your distribution, kernel and architecture. For this example I used CentOS 4.3, so the following packages were required.

Install the packages as the root user.

# rpm -Uvh oracleasm-support-2.0.1-1.i386.rpm \
           oracleasmlib-2.0.1-1.i386.rpm \
           oracleasm-2.6.9-34.ELsmp-2.0.1-1.i686.rpm
Preparing...                ########################################### [100%]
   1:oracleasm-support      ########################################### [ 33%]
   2:oracleasm-2.6.9-34.ELsm########################################### [ 67%]
   3:oracleasmlib           ########################################### [100%]
#

With the software installed, configure the ASM kernel module.

# /etc/init.d/oracleasm configure
Configuring the Oracle ASM library driver.

This will configure the on-boot properties of the Oracle ASM library
driver.  The following questions will determine whether the driver is
loaded on boot and what permissions it will have.  The current values
will be shown in brackets ('[]').  Hitting <ENTER> without typing an
answer will keep that current value.  Ctrl-C will abort.

Default user to own the driver interface []: oracle
Default group to own the driver interface []: oinstall
Start Oracle ASM library driver on boot (y/n) [n]: y
Fix permissions of Oracle ASM disks on boot (y/n) [y]:
Writing Oracle ASM library driver configuration:           [  OK  ]
Creating /dev/oracleasm mount point:                       [  OK  ]
Loading module "oracleasm":                                [  OK  ]
Mounting ASMlib driver filesystem:                         [  OK  ]
Scanning system for ASM disks:                             [  OK  ]
#

Once the kernel module is loaded, stamp (or label) the partitions created earlier as ASM disks.

# /etc/init.d/oracleasm createdisk VOL1 /dev/sdb1
Marking disk "/dev/sdb1" as an ASM disk:                   [  OK  ]
# /etc/init.d/oracleasm createdisk VOL2 /dev/sdc1
Marking disk "/dev/sdc1" as an ASM disk:                   [  OK  ]
# /etc/init.d/oracleasm createdisk VOL3 /dev/sdd1
Marking disk "/dev/sdd1" as an ASM disk:                   [  OK  ]
#

If this were a RAC installation, the disks would only be stamped by one node. The other nodes would just scan for the disks.

# /etc/init.d/oracleasm scandisks
Scanning system for ASM disks:                             [  OK  ]
#

The stamped disks are listed as follows.

# /etc/init.d/oracleasm listdisks
VOL1
VOL2
VOL3
#

The disks are now ready to be used by ASM.

Raw Device Setup

This step is only necessary if you want ASM to access the disks as raw devices.

Edit the "/etc/sysconfig/rawdevices" file, adding the following lines.

/dev/raw/raw1 /dev/sdb1
/dev/raw/raw2 /dev/sdc1
/dev/raw/raw3 /dev/sdd1

Restart the rawdevices service using the following command.

service rawdevices restart

Run the following commands and add them the "/etc/rc.local" file.

chown oracle:oinstall /dev/raw/raw1
chown oracle:oinstall /dev/raw/raw2
chown oracle:oinstall /dev/raw/raw3
chmod 600 /dev/raw/raw1
chmod 600 /dev/raw/raw2
chmod 600 /dev/raw/raw3

The ASM raw device disks are now configured.

ASM Creation

Creation of the ASM instance is the same, regardless of the use of ASMLib or raw devices. When using ASMLib, the candidate disks are listed using the stamp associated with them, while the raw devices are listed using their device name.

To configure an ASM instance, start the Database Configuration Assistant by issuing the "dbca" command as the oracle user. On the "Welcome" screen, click the "Next" button.

DBCA Welcome

Select the "Configure Automatic Storage Management" option, then click the "Next" Button.

DBCA Configure ASM

If the Oracle Cluster Syncronization Service (CSS) is not currently running, a warning screen will be displayed. Follow the instructions and click the "OK" button. Once you've returned to the previous screen, click the "Next" button again.

DBCA CSS Warning

The script gives the following output.

# /u01/app/oracle/product/10.2.0/db_1/bin/localconfig add
/etc/oracle does not exist. Creating it now.
Successfully accumulated necessary OCR keys.
Creating OCR keys for user 'root', privgrp 'root'..
Operation successful.
Configuration for local CSS has been initialized

Adding to inittab
Startup will be queued to init within 90 seconds.
Checking the status of new Oracle init process...
Expecting the CRS daemons to be up within 600 seconds.
CSS is active on these nodes.
        centos2
CSS is active on all nodes.
Oracle CSS service is installed and running under init(1M)
#

Enter a password for the ASM instance, then click the "Next" button.

DBCA ASM Password

On the confirmation screen, click the "OK" button.

DBCA ASM Creation Confirm

Wait while the ASM instance is created.

DBCA ASM Creation

Once the ASM instance is created, you are presented with the "ASM Disk Groups" screen. Click the "Create New" button.

DBCA ASM Disk Groups

On the "Create Disk Group" screen, enter Disk Group Name of "DATA" and select the required level of redundancy:

In this example, the "High" redundancy is used. Select all three candidate disks and click the "OK" button. The following image shows how the candidate disks are displayed when using ASMLib.

DBCA Create Disk Group ASMLib

When using raw devices, the candidate discs are listed using the devide names.

DBCA Create Disk Group Raw Devices

On the "ASM Disk Groups" screen. Click the "Finish" button.

DBCA ASM Disk Groups

Click the "Yes" button to perform another operation.

DBCA Another Operation

You are now ready to create a database instance using ASM.

Database Creation

Before continuing with the database creation, check the listener is up and the ASM instance has registered with it. Start the listener using the following command.

$ lsnrctl start

LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 29-APR-2006 14:35:46

Copyright (c) 1991, 2005, Oracle.  All rights reserved.

Starting /u01/app/oracle/product/10.2.0/db_1/bin/tnslsnr: please wait...

TNSLSNR for Linux: Version 10.2.0.1.0 - Production
Log messages written to /u01/app/oracle/product/10.2.0/db_1/network/log/listener .log
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=centos2.localdomain)(POR T=1521)))

Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 10.2.0.1.0 - Production
Start Date                29-APR-2006 14:35:47
Uptime                    0 days 0 hr. 0 min. 0 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Log File         /u01/app/oracle/product/10.2.0/db_1/network/log/listen er.log
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=centos2.localdomain)(PORT=1521)))
The listener supports no services
The command completed successfully
$

The ASM instance is not registered, so we can force the registration by doing the following.

$ export ORACLE_SID=+ASM
$ sqlplus / as sysdba

SQL*Plus: Release 10.2.0.1.0 - Production on Sat Apr 29 14:37:06 2006

Copyright (c) 1982, 2005, Oracle.  All rights reserved.


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

SQL> alter system register;

System altered.

SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Pr oduction
With the Partitioning, OLAP and Data Mining options
$

Checking the status of the listener shows that the ASM instance is now registered.

$ lsnrctl status

LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 29-APR-2006 14:37:32

Copyright (c) 1991, 2005, Oracle.  All rights reserved.

Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 10.2.0.1.0 - Production
Start Date                29-APR-2006 14:35:47
Uptime                    0 days 0 hr. 1 min. 46 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Log File         /u01/app/oracle/product/10.2.0/db_1/network/log/listen er.log
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=centos2.localdomain)(PORT=1521)))
Services Summary...
Service "+ASM" has 1 instance(s).
  Instance "+ASM", status BLOCKED, has 1 handler(s) for this service...
Service "+ASM_XPT" has 1 instance(s).
  Instance "+ASM", status BLOCKED, has 1 handler(s) for this service...
The command completed successfully
$

Go back to the DBCA and create a custom database in the normal way, selecting the "Automatic Storage Management (ASM)" storage option.

DBCA ASM Storage Option

Enter the ASM password if prompted, then click the "OK" button.

DBCA ASM Password

Select the "DATA" disk group, then clicking the "Next" button.

DBCA ASM Disk Groups

Accept the default "Oracle-Managed Files" database location by clicking the "Next" button.

DBCA Database Location

Enable the "Flash Recovery Area" and Archiving, using the "+DATA" disk group for both.

DBCA Recovery Config

Continue with the rest of the DBCA, selecting the required options along the way.

Switching from Raw Devices to ASMLib

Shutdown any databases using the ASM instance, but leave the ASM instance itself running. Connect to the running ASM instance.

$ export ORACLE_SID=+ASM
$ sqlplus / as sysdba

Perform the ASMLib Installation, but stop prior to stamping the ASM disk. If you attempt to stamp the disks using the createdisk command it will fail.

Alter the ASM disk discovery string to exclude the raw devices used previously, then shutdown the ASM instance.

SQL> ALTER SYSTEM SET asm_diskstring = 'ORCL:VOL*' SCOPE=SPFILE;

System altered.

SQL> SHUTDOWN IMMEDIATE;
ASM diskgroups dismounted
ASM instance shutdown
SQL>

If you are planning to remove the raw device mappings (Raw Device Setup), you could simply reset the ASM_DISKGROUP parameter.

SQL> ALTER SYSTEM RESET asm_diskstring SCOPE=SPFILE SID='*';

System altered.

SQL>

At this point the disks will not be used by ASM because they are not stamped. As mentioned previously, the createdisk command used to stamp new disks would fail, so we must issue the renamedisk command as the root user for each disk.

# /etc/init.d/oracleasm renamedisk /dev/sdb1 VOL1
Renaming disk "/dev/sdb1" to "VOL1":                       [  OK  ]
# /etc/init.d/oracleasm renamedisk /dev/sdc1 VOL2
Renaming disk "/dev/sdc1" to "VOL2":                       [  OK  ]
# /etc/init.d/oracleasm renamedisk /dev/sdd1 VOL3
Renaming disk "/dev/sdd1" to "VOL3":                       [  OK  ]
#

Notice, the stamp matches the discovery string set earlier. The ASM instance can now be started.

SQL> STARTUP
ASM instance started

Total System Global Area   83886080 bytes
Fixed Size                  1217836 bytes
Variable Size              57502420 bytes
ASM Cache                  25165824 bytes
ASM diskgroups mounted
SQL>

The ASM instance is now using ASMLib, rather than raw devices. All dependent databases can now be started.

Switching from ASMLib to Raw Devices

Shutdown any databases using the ASM instance, but leave the ASM instance itself running. Connect to the running ASM instance.

$ export ORACLE_SID=+ASM
$ sqlplus / as sysdba

Alter the ASM disk discovery string to match the raw devices you plan to set up, then shutdown the ASM instance.

SQL> ALTER SYSTEM SET asm_diskstring = '/dev/raw/raw*' SCOPE=SPFILE;

System altered.

SQL> SHUTDOWN IMMEDIATE;
ASM diskgroups dismounted
ASM instance shutdown
SQL>

Perform all the steps listed in the Raw Device Setup, then start the ASM instance.

SQL> STARTUP
ASM instance started

Total System Global Area   83886080 bytes
Fixed Size                  1217836 bytes
Variable Size              57502420 bytes
ASM Cache                  25165824 bytes
ASM diskgroups mounted
SQL>

The ASM instance is now using the disks as raw devices, rather than as ASMLib disks. All dependent databases can now be started.

Performance Comparison

Some documents suggests using ASMLib with Oracle 10g Release 2 gives superior disk performance, while others say it only reduces the time searching for candidate disks, and hence ASM startup time. I decided to compare the performance of the two methods myself to see if I could tell the difference.

My first thought was to perform a simple insert/update/delete test, so I created the following user and schema for the test in a database using and ASM instance using ASMLib.

export ORACLE_SID=DB10G
sqlplus / as sysdba

CREATE TABLESPACE test_ts;

CREATE USER test_user IDENTIFIED BY test_user DEFAULT TABLESPACE test_ts QUOTA UNLIMITED ON test_ts;
GRANT CONNECT, CREATE TABLE TO test_user;

CONN test_user/test_user

CREATE TABLE test_tab (
  id    NUMBER,
  data  VARCHAR2(4000),
  CONSTRAINT test_tab_pk PRIMARY KEY (id)
);

Then, as the test user, I ran the following code several times and calculated an average time for each operation.

SET SERVEROUTPUT ON
DECLARE
  l_loops NUMBER := 1000;
  l_data  VARCHAR2(32767) := RPAD('X', 4000, 'X');
  l_start NUMBER;
BEGIN
  l_start := DBMS_UTILITY.get_time;

  FOR i IN 1 .. l_loops LOOP
    INSERT INTO test_tab (id, data) VALUES (i, l_data);
    COMMIT;
  END LOOP;

  DBMS_OUTPUT.put_line('Inserts (' || l_loops || '): ' || (DBMS_UTILITY.get_time - l_start) || ' hsecs');

  l_start := DBMS_UTILITY.get_time;

  FOR i IN 1 .. l_loops LOOP
    UPDATE test_tab
    SET    data = l_data
    WHERE  id   = i;
    COMMIT;
  END LOOP;

  DBMS_OUTPUT.put_line('Updates (' || l_loops || '): ' || (DBMS_UTILITY.get_time - l_start) || ' hsecs');

  l_start := DBMS_UTILITY.get_time;

  FOR i IN 1 .. l_loops LOOP
    DELETE FROM test_tab
    WHERE  id = i;
    COMMIT;
  END LOOP;

  DBMS_OUTPUT.put_line('Deletes (' || l_loops || '): ' || (DBMS_UTILITY.get_time - l_start) || ' hsecs');

  EXECUTE IMMEDIATE 'TRUNCATE TABLE test_tab';
END;
/

The code is purposely inefficient, using a single statement and a commit within a loop for each operation. Remember, the ASM instance is using high redundancy, so each physical write operation is effectively done 3 times.

Once the tests on ASMLib were complete, I switched to using raw devices and repeated the tests. The average results for 1000 of each operation are listed below.

Operation       ASMLib (hsecs)  Raw Devices (hsecs)
==============  ==============  ===================
Inserts (1000)             468                  852
Updates (1000)             956                 1287
Deletes (1000)            1281                 1995

You can instantly see that the ASMLib results are better than those of the raw devices, but the testing is suspect for the following reasons:

For these reasons, I decided not to continue to persue the performance tests, much to the delight of my hard drive. If I get access to a more realistic setup I will attempt some large scale tests and report the outcome.

For more information see:

Hope this helps. Regards Tim...

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