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 Oracle ASM using ASMLib
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.

Once ASMlib is completely configured, the root user can apply the /etc/init.d/oracleasm script to create new ASM disks using block devices.  Other nodes in the cluster can then scan for these disks and become aware of them. In order to label disks as ASM disks, the following command can be used:

 

/etc/init.d/oracleasm createdisk DISKNAME /dev/path

  • DISKNAME - The ASM label for the disk.  The name chosen does not matter, but it is a good idea to choose names logically that will help with management down the road.

  • /dev/path - The path to the block device to be labeled.  For instance, /dev/sdb1 is a block device that ASM could label for its own use.

Figure 3.2:  Creating ASM Disks with Oracleasm

After the disks have been created, verify the disk labels with the listdisks parameter:

 

# /etc/init.d/oracleasm listdisks

DATA1

DATA2

DATA3

 

It is a good idea to get sysadmins used to the idea of querying disks to make sure they are not being used for ASM before performing any possibly harmful actions against the system's disks.  The querydisk argument can be used for this purpose:

 

/etc/init.d/oracleasm querydisk /dev/path

  • /dev/path - A block device to query for the presence of an ASM label.

Figure 3.3:  Querying Disks with Oracleasm

If the queried device is not an ASM disk, then FAILED will be displayed between the brackets.

 

In a RAC configuration, all nodes must be able to see the same ASM devices.  The good news is that one does not have to go through the creation of disks with ASMlib on each system in the RAC cluster.  However, one must install ASMlib on each node.

 

Once disks have been created on one node, the other nodes can be updated using the /etc/init.d/oracleasm scandisks function.  Using the scandisks argument will instruct oracleasm to browse the /dev tree to find devices which have been labeled for use by ASMlib.

 

Figure 3.4:  Scanning Disks on Other Nodes with Oracleasm

Note: Even though disks have been tagged as being ASM disks, one must still add the disks into an ASM diskgroup as described in an upcoming section.

Windows

On Windows, ASM disks will incorporate logical drives set up using the Windows Disk Management tool (Start à All Programs à Administrative Tools à Computer Management).  These drives should contain no filesystem and should not be assigned a drive letter.

 

ASM can be configured during the installation of Oracle 11g or later through DBCA.  In Windows, the Stamp Disks button allows the DBA to select logical drives that should be used for ASM.  Much like ASMlib, disks that are available as candidate devices can be labeled as ASM drives.

 

Note: While stamping disks, one may notice the OCR and voting disks in the list.  These disks will have a status of “Oracle raw device file” while available disks will say “Candidate device.”

 

Once disks have been stamped as ASM candidates, it will be possible to add them to an ASM diskgroup.  All disks will have a prefix of \\.\ORCLDISK plus any prefix added while stamping.  For instance, during labeling, if the DBA elects to use a prefix of FLASH, labeled disks will take the format \\.\ORCLDISKFLASH# where # is an incrementing number.

Other OS – HPUX, AIX, Solaris

Each OS has its own way of representing disks.  In HPUX, AIX, and SOLARIS, there is usually a block device and character device pair that can be used.

 

On Solaris, for instance, two directories exist in the /dev tree:

  • /dev/dsk

  • /dev/rdsk

The /dev/dsk location is known as a block device: a single slice or partition of a disk.  /dev/rdsk, on the other hand, is a character device, which is the logical representation of the slice. When creating ASM disks, these character devices will be referenced.  The location of character devices depends upon the OS:

  • Solaris -  /dev/rdsk

  • HPUX -  /dev/rdsk

  • HPUX(True 64) -  /dev/rdisk

  • AIX -  /dev/rhdisk

Note: Multipathing can change the location of character devices depending upon the system used.  Consult the specific documentation to find the path to use for character devices.

Symbolic Links

Symbolic links can be used to ease management.  For example:

 

ln -s /dev/sdb1 /dev/ocr

ln -s /dev/sdb2 /dev/voting

 

This allows logical names to be referenced during setup and makes changing paths/devices easier.

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