Oracle ASM Disk Discovery

Disk discovery is the mechanism used to find the operating system names for disks Oracle ASM can access.

This mechanism finds all the disks that comprise a disk group to be mounted, the disks an administrator wants to add to a disk group, or the disks the administrator might consider adding to a disk group. This section contains the following topics:

How A Disk is Discovered

When an Oracle ASM instance is initialized, Oracle ASM discovers and examines the contents of all of the disks that are in the paths that you designated with values in the ASM_DISKSTRING initialization parameter.

Disk discovery also occurs when you:

Run the following SQL statements
- Mount a disk group with ALTER DISKGROUP MOUNT
- Online a disk with ALTER DISKGROUP ONLINE DISK
- Add a disk to a disk group with CREATE or ALTER DISKGROUP...ADD DISK
- Resize a disk in a disk group with ALTER DISKGROUP...RESIZE DISK
- Query with SELECT FROM V$ASM_DISKGROUP or V$ASM_DISK views
Run Oracle ASM Configuration Assistant (ASMCA) operations or ASMCMD commands that perform the same operations as the SQL statements previously listed

After Oracle ASM successfully discovers a disk, the disk appears in the V$ASM_DISK view. Disks that belong to a disk group, that is, disks that have a disk group name in the disk header, show a header status of MEMBER. Disks that were discovered, but that have not yet been assigned to a disk group, have a status of either CANDIDATE or PROVISIONED. Disks that previously belonged to a disk group and were dropped cleanly from the disk group have a status of FORMER.

The PROVISIONED status implies that an additional platform-specific action has been taken by an administrator to make the disk available for Oracle ASM. For example, on Windows computers, the administrator might have used asmtool or asmtoolg to stamp the disk with a header. For example, on Linux computers, the administrator might have used Oracle ASM Filter Driver or ASMLIB to prepare the disk for Oracle ASM.

Example 4-10 shows a SQL query on V$ASM_DISK that displays the header status of a group of disks.

Example 4-10 Querying V$ASM_DISK for header status

SQL> SELECT name, header_status, path FROM V$ASM_DISK 
     WHERE path LIKE '/devices/disk0%';

NAME      HEADER_STATUS PATH
--------- ------------- ---------------------
          FORMER        /devices/disk02
          FORMER        /devices/disk01
          CANDIDATE     /devices/disk07
DISK06    MEMBER        /devices/disk06
DISK05    MEMBER        /devices/disk05
DISK04    MEMBER        /devices/disk04
DISK03    MEMBER        /devices/disk03
7 rows selected.

Disk Discovery Rules

The rules for discovering Oracle ASM disks are as follows:

Oracle ASM can discover up to 10,000 disks. That is, if more than 10,000 disks match the ASM_DISKSTRING initialization parameter, then Oracle ASM discovers only the first 10,000.
Oracle ASM only discovers disk partitions. Oracle ASM does not discover partitions that include the partition table.
From the perspective of the installation, candidate disks are those that have the CANDIDATE, PROVISIONED, or FORMER header status. These disks with a CANDIDATE, PROVISIONED, or FORMER status can be added to Oracle ASM disk groups without using the FORCE flag.
When adding a disk, the FORCE option must be used if Oracle ASM recognizes that the disk was managed by Oracle. Such a disk appears in the V$ASM_DISK view with a status of FOREIGN. In this case, you can only add the disk to a disk group by using the FORCE keyword.
MEMBER disks can usually be added to a disk group by specifying the FORCE flag, if the disks are not part of a currently mounted disk group.

In addition, Oracle ASM identifies the following configuration errors during discovery:

Multiple paths to the same disk

In this case, if the disk is part of a disk group, then disk group mount fails. If the disk is being added to a disk group with the ADD DISK or CREATE DISKGROUP command, then the command fails. To correct the error, adjust the ASM_DISKSTRING value so that Oracle ASM does not discover multiple paths to the same disk. Or if you are using multipathing software, then ensure that you include only the pseudo-device name in the ASM_DISKSTRING value. See "Oracle ASM and Multipathing".
Multiple Oracle ASM disks with the same disk header

This can be caused by having copied one disk onto another. In this case, the disk group mount operation fails.

Improving Disk Discovery Time

The value for the ASM_DISKSTRING initialization parameter is an operating system–dependent value that Oracle ASM uses to limit the set of paths that the discovery process uses to search for disks. When a new disk is added to a disk group, each Oracle ASM instance that has the disk group mounted must be able to discover the new disk using its ASM_DISKSTRING.

In many cases, the default value (NULL) is sufficient. Using a more restrictive value might reduce the time required for Oracle ASM to perform discovery, and thus improve disk group mount time or the time for adding a disk to a disk group. Oracle may dynamically change the ASM_DISKSTRING before adding a disk so that the new disk is discovered through this parameter.

The default value of ASM_DISKSTRING might not find all disks in all situations. If your site is using Oracle ASM Filter Driver or a third-party vendor, then you may have discovery string conventions that you must use for ASM_DISKSTRING.

In addition, if your installation uses multipathing software, then the software might place pseudo-devices in a path that is different from the operating system default.