ORACLE 参量之ASM_DISKSTRING

ASM_DISKSTRING

TheASM_DISKSTRINGinitialization parameter specifies a comma-delimited list of strings that limits the set of disks that an Oracle ASM instance discovers. The discovery strings can include wildcard characters. Only disks that match one of the strings are discovered. The same disk cannot be discovered twice.

The discovery string format depends on the Oracle ASM library and the operating system that are in use. Pattern matching is supported. Refer to your operating system-specific installation guide for information about the default pattern matching.

For example, on a Linux server that does not use ASMLib, to limit the discovery process to only include disks that are in the/dev/rdsk/mydisksdirectory, set theASM_DISKSTRINGinitialization parameter to:

/dev/rdsk/mydisks/*

The asterisk is required. To limit the discovery process to only include disks that have a name that ends indisk3ordisk4, setASM_DISKSTRINGto:

/dev/rdsk/*disk3,/dev/rdsk/*disk4

The?character, when used as the first character of a path, expands to the Oracle home directory. Depending on the operating system, when you use the?character elsewhere in the path, it is a wildcard for one character.

The default value of theASM_DISKSTRINGparameter is aNULLstring. ANULLvalue causes Oracle ASM to search a default path for all disks in the system to which the Oracle ASM instance has read and write access. The default search path is platform-specific. Refer to your operating system specific installation guide for more information about the default search path.

Oracle ASM cannot use a disk unless all of the Oracle ASM instances in the cluster can discover the disk through one of their own discovery strings. The names do not have to be the same on every node, but all disks must be discoverable by all of the nodes in the cluster. This may require dynamically changing the initialization parameter to enable adding new storage.

For additional information about discovering disks, see"Oracle ASM Disk Discovery".

See Also:

• Oracle Exadata documentationfor information about the Oracle ASM discovery string format for Oracle Exadata

• Oracle Database Referencefor more information about theASM_DISKSTRINGinitialization parameter

Oracle ASM Disk Discovery

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

• Disk Discovery Rules

• Improving Disk Discovery Time

For additional information about disk discovery and theASM_DISKSTRINGinitialization parameter, refer to"ASM_DISKSTRING".

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 theASM_DISKSTRINGinitialization parameter.

Disk discovery also occurs when you:

• Run the following SQL statements

  • Mount a disk group withALTERDISKGROUP...MOUNT

  • Online a disk withALTERDISKGROUP...ONLINEDISK

  • Add a disk to a disk group withCREATEorALTERDISKGROUP...ADDDISK

  • Resize a disk in a disk group withALTERDISKGROUP...RESIZEDISK

  • Query withSELECT...FROMV$ASM_DISKGROUPorV$ASM_DISKviews

• Run Oracle Enterprise Manager or Oracle ASM Configuration Assistant (ASMCA) operations that invoke the SQL statements previously listed

• Run ASMCMD commands that perform the same operations as the SQL statements previously listed

After Oracle ASM successfully discovers a disk, the disk appears in theV$ASM_DISKview. Disks that belong to a disk group, that is, disks that have a disk group name in the disk header, show a header status ofMEMBER. Disks that were discovered, but that have not yet been assigned to a disk group, have a status of eitherCANDIDATEorPROVISIONED. Disks that previously belonged to a disk group and were dropped cleanly from the disk group have a status ofFORMER.

ThePROVISIONEDstatus 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 usedasmtoolorasmtoolgto stamp the disk with a header. On Linux computers, the administrator might have used ASMLib to prepare the disk for Oracle ASM.

Example 4-10shows a SQL query onV$ASM_DISKthat 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.

See Also:

Oracle Database Referencefor information about the header status of an Oracle ASM disk that is displayed in theV$ASM_DISKview

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 theASM_DISKSTRINGinitialization 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 theCANDIDATE,PROVISIONED, orFORMERheader status. These disks with aCANDIDATE,PROVISIONED, orFORMERstatus can be added to Oracle ASM disk groups without using theFORCEflag.

• When adding a disk, theFORCEoption must be used if Oracle ASM recognizes that the disk was managed by Oracle. Such a disk appears in theV$ASM_DISKview with a status ofFOREIGN. In this case, you can only add the disk to a disk group by using theFORCEkeyword.

• MEMBERdisks can usually be added to a disk group by specifying theFORCEflag, 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 theADD DISKorCREATE DISKGROUPcommand, then the command fails. To correct the error, adjust theASM_DISKSTRINGvalue 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 theASM_DISKSTRINGvalue. 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 theASM_DISKSTRINGinitialization 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 itsASM_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 theASM_DISKSTRINGbefore adding a disk so that the new disk is discovered through this parameter.

The default value ofASM_DISKSTRINGmight not find all disks in all situations. If your site is using a third-party vendor ASMLib, then the vendor might have discovery string conventions that you must use forASM_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. See"Oracle ASM and Multipathing"and consult the multipathing vendor documentation for details.

Q131.

When starting up your ASM instance, you receive the followingerror:

What is the cause of the error?

A. The ASM_DISKGROUPS parameter is configured for three diskgroups: DGROUP1, DGROUP2, and DGROUP3. The underlying disks for these diskgroups have apparently been lost.

B. The format of the ASM_DISKGROUPS parameter is incorrect.It should reference the disk group numbers, not the names of the disk groups

C. The ASM_POWER_LIMIT parameter is incorrectly set to 1. Itshould be set to the number of disk groups being attached to the ASM instance.

D. The ASM_DISKSTRING parameter is not set; therefore 因此disk discovery is not possible.

E. There is insufficient information to solve this problem.

Answer: D