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 Creating ASM Disk Groups
Oracle Tips by Burleson Consulting

Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Using OUI or DBCA, ASM instances can be graphically created over the RAC cluster.  It is also possible to create the instances manually by creating ASM initialization parameter files and adding the instances to the cluster.

 

Once ASM has been configured, adding diskgroups can be done in DBCA, in Enterprise Manager, or at the SQL prompt.  Using DBCA is extremely simple; the DBA must enter a name, a redundancy option, and choose the disks that will be part of the ASM diskgroup.

 

Creating ASM diskgroups at the SQL prompt is much the same, but the selections must be made through commands.  For example:

 

CREATE DISKGROUP DATA

  EXTERNAL REDUNDANCY

  DISK 'ORCL:DATA1', 'ORDL:DATA2';

 

This command will create a diskgroup called DATA that contains the disks marked DATA1 and DATA2 by ASMlib.

 

It is also possible to specify a search string for the location of the disks:

 

CREATE DISKGROUP DATA

  EXTERNAL REDUNDANCY

  DISK '/dev/rdsk/*';

 

Redundancy

 

In the above examples, the EXTERNAL REDUNDANCY clause was used.  This means that ASM will provide no extra redundancy support and, therefore, cannot help in the case of disk failure.  This is usually an option only when using RAID 1 (mirroring).  However, redundancy can be set up within ASM using failure groups.

 

Failure groups allow a DBA to specify two pools of disks that can hold copies of each other's data.  ASM allows NORMAL redundancy, in which a diskgroup is composed of two failgroups, or HIGH redundancy, in which a diskgroup is composed of three failgroups.

 

When files are written to the ASM diskgroup under NORMAL redundancy, the files will be written to both failgroups in a round-robin fashion. Files are read from the primary failgroup.  For example:

 

CREATE DISKGROUP DATA

  NORMAL REDUNDANCY

  FAILGROUP failgrp1 DISK

    '/dev/sdb1', '/dev/sdb2'

  FAILGROUP failgrp2 DISK

    '/dev/sdc1', '/dev/sdc2';

 

In this example, a file will first go to failgrp1 and be copied to failgrp2.  The next file will be written to failgrp2 and copied to failgrp1.

 

In Oracle 11g, the DBA can also specify Fast Failure Repair options.  This means that if a disk in a failgroup is damaged, Oracle can repair the damaged portion of the disk instead of cloning all files to the disk from scratch.  A repair time must be set as an attribute of an ASM diskgroup, specifying how long Oracle should keep the information necessary to rebuild a damaged disk.  For example:

 

ALTER DISKGROUP DATA SET ATTRIBUTE 'disk_repair_time' = '3H';

 

This command will change the maximum repair time of the DATA diskgroup to three hours, as opposed to the default of 3.6.

 

Another new feature of 11g AS< is preferred read groups.  An ASM parameter called asm_preferred_read_failure_groups can be set per instance.  This means that one instance in a two-node cluster can read primarily from one set of mirrored disks, and the other instance can read from another set of mirrored disks.

 

There are other new features of ASM in Oracle 11g that are outside the scope of this book.  For more information about these features, please consult Oracle 11g New Features by Rampant Tech Press.

RAC Using NFS with Direct NFS (DNFS)

Oracle 11g comes with enhanced support for Oracle over NFS using the new Direct NFS feature.  Direct NFS allows for costs savings by sticking with one connection model: the network.  This allows for multipathing and unified storage.  In addition, Direct NFS even works in Windows, even though Windows has no NFS support. Another nice feature of NFS is that files are directly accessible via standard OS commands like ls, mv, cp, and such.

 

Lastly, Oracle's Direct NFS feature allows for Direct I/O and Asynchronous I/O by default. To use Direct NFS, the client must be installed on RAC nodes.  This can even be performed on nodes where Oracle RAC is already running.

 

The Oracle 11g Direct NFS Client is shipped with Oracle 11g.  Once installed, it is very simple to put it in place:

Stop any RAC databases

  1. Navigate to $ORACLE_HOME/lib

  2. Move the libodm11.so file to a backup

  3. Create a symbolic link called libodm11.so which uses libnfsodm11.so as its source

ln -s libfsodm11.so libodm11.so

  1. Start the RAC databases

Direct NFS information can then be queried from the following fixed views:

  • v$dnfs_channels

  • v$dnfs_files

  • v$dnfs_servers

  • v$dnfs_stats

Note: Oracle 11g Direct NFS only works with NFS V3 compatible NAS devices.

 

NFS must be set up at the OS level before using the Direct NFS client.  Each OS has different requirements for configuring and using NFS.

Conclusion

There are many options for data storage in a RAC environment.  This is much different from the days where the only options were raw volumes or third party cluster file systems.  With these options, it is possible for the DBA and System Administrators to work together to find an optimal environment for their RAC cluster.

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