Disk groups that are specified in the ASM_DISKGROUPS initialization parameter are mounted automatically at Oracle ASM instance startup. This process makes the disk groups available to all database instances running on the same node as Oracle ASM. The disk groups are dismounted at Oracle ASM instance shutdown. Oracle ASM also automatically mounts a disk group when you initially create it, and dismounts a disk group if you drop it.

When a disk group is mounted, a disk group number is chosen. This number may change across disk group mounts. A disk group number is not recorded in any persistent structure, but the current value can be viewed in the GROUP_NUMBER column of the V$ASM views.

When you want to mount or dismount disk groups manually, use the ALTER DISKGROUP...MOUNT or ALTER DISKGROUP...DISMOUNT statement. You can mount or dismount disk groups by name, or specify ALL. You can also specify the optional FORCE option. For information about using the FORCE option, refer to Mounting Disk Groups Using the FORCE Option.

If you try to dismount a disk group that contains open files, the statement fails, unless you also specify the FORCE clause.

In a clustered Oracle ASM environment in RESTRICTED mode, a disk group is mounted in single-instance exclusive mode. No other Oracle ASM instance in that cluster can mount that disk group. In this mode the disk group is not usable by any Oracle ASM client. Use this mode to perform a fast rebalance.

The following SQL statement dismounts all disk groups that are currently mounted to the Oracle ASM instance:

ALTER DISKGROUP ALL DISMOUNT;

The following SQL statement mounts disk group data1:

ALTER DISKGROUP data1 MOUNT;

Oracle ASM provides a MOUNT FORCE option with ALTER DISKGROUP to enable Oracle ASM disk groups to be mounted in normal or high redundancy modes even though some Oracle ASM disks may be unavailable to the disk group at mount time.

The default behavior without the FORCE option is to fail to mount a disk group that has damaged or missing disks.

The MOUNT FORCE option is useful in situations where a disk is temporarily unavailable and you want to mount the disk group with reduced redundancy while you correct the situation that caused the outage.

To successfully mount with the MOUNT FORCE option, Oracle ASM must be able to find at least one copy of the extents for all of the files in the disk group. In this case, Oracle ASM can successfully mount the disk group, but with potentially reduced redundancy.

The disks that Oracle ASM cannot access are placed in an offline mode. Oracle ASM then begins timing the period that these disks are in an offline mode. If the disk offline time period exceeds the timer threshold set by DISK_REPAIR_TIME disk group attribute, then those disks are permanently dropped from the disk group. You can change the offline timer after a disk is put in an offline state by using the ALTER DISKGROUP OFFLINE statement.

In clustered Oracle ASM environments, if an Oracle ASM instance is not the first instance to mount the disk group, then using the MOUNT FORCE statement fails. This is because the disks have been accessed by another instance and the disks are not locally accessible.

If all disks are available, then using the FORCE option causes the MOUNT command to fail. This discourages unnecessary and improper use of the feature.

The following example shows how to use the FORCE option to force the mount of the data1 disk group:

SQL> ALTER DISKGROUP data1 MOUNT FORCE;

You can check the internal consistency of disk group metadata using the ALTER DISKGROUP statement with the CHECK keyword. You can use this statement to check specific files in a disk group, specific disks or all disks in a disk group, or specific failure groups within a disk group. The disk group must be mounted to perform these checks.

By default, the CHECK DISK GROUP clause verifies all of the metadata directories. Oracle ASM displays summary errors and writes the details about the errors in an alert log. The CHECK keyword performs the following operations:

• Verifies the consistency of the disk

• Cross checks all of the file extent maps and allocation tables for consistency

• Checks that the alias metadata directory and file directory are linked correctly

• Verifies that the alias directory tree is linked correctly

• Checks that Oracle ASM metadata directories do not have unreachable allocated blocks

The REPAIR | NOREPAIR clause specifies whether Oracle ASM should attempt to repair errors that are found during the check. The default is NOREPAIR. Use the NOREPAIR clause to receive alerts about inconsistencies and to suppress Oracle ASM from resolving the errors automatically. The following example statement checks for consistency in the metadata for all disks in the data1 disk group:

ALTER DISKGROUP data1 CHECK ALL;

The DROP DISKGROUP statement enables you to delete an Oracle ASM disk group and optionally, all of its files.

You can specify the INCLUDING CONTENTS clause if you also want to delete all files that are contained in the disk group. The default is EXCLUDING CONTENTS, which provides syntactic consistency and prevents you from dropping the disk group if it has any contents

The Oracle ASM instance must be started and the disk group must be mounted with none of the disk group files open, in order for the DROP DISKGROUP statement to succeed. The statement does not return until the disk group has been dropped.

When you drop a disk group, Oracle ASM dismounts the disk group and removes the disk group name from the ASM_DISKGROUPS initialization parameter if a server parameter file is being used. If a text initialization parameter file is being used, and the disk group is mentioned in the ASM_DISKGROUPS initialization parameter, then you must remove the disk group name from the ASM_DISKGROUPS initialization parameter before the next time that you shut down and restart the Oracle ASM instance.

The following statement deletes data1:

DROP DISKGROUP data1;

After ensuring that none of the files contained in data1 are open, Oracle ASM rewrites the header of each disk in the disk group to remove Oracle ASM formatting information. The statement does not specify INCLUDING CONTENTS, so the drop operation fails if the disk group contains any files.

If an Oracle Automatic Storage Management Cluster File System (Oracle ACFS) file system is mounted on a volume contained in the disk group, then the file system must be dismounted. If the file system has been registered, then it must be deregistered. The INCLUDING CONTENTS clause must be used to drop this disk group. All data in the file system is destroyed. To view the volumes and mount paths associated with a disk group, you can query the V$ASM_VOLUME view.

If you cannot mount a disk group but must drop it, you can use the FORCE option of the DROP DISKGROUP statement. This command enables you to remove the headers on disks that belong to a disk group that cannot be mounted by any Oracle ASM instances as shown in the following example:

SQL> DROP DISKGROUP data1 FORCE;

The disk group on which you perform this operation should not be mounted anywhere in the cluster. When you use the FORCE option, the Oracle ASM instance does not attempt to verify that a disk group is being used by another Oracle ASM instance in the same storage subsystem.

You can also drop disks from a disk group with ASMCA.

The renamedg tool enables you to change the name of a cloned disk group. The disk group must be dismounted on all nodes in the cluster before running renamedg on the disk group.

renamedg renames a disk group using a two-step process:

1. Phase one

   This phase generates a configuration file to be used in phase two.

2. Phase two

   This phase uses the configuration file to perform the renaming of the disk group.

The syntax is:

renamedg {-help | help=true}

renamedg
     [phase={one|two |both}] dgname=diskgroup 
       newdgname=newdiskgroup [config=configfile]
     [asm_diskstring=discoverystring, discoverystring ...] 
     [clean={true|false}] [check={true|false}]
     [confirm={true|false}] [verbose={ true|false}]
     [keep_voting_files={true|false}]

• phase={one|two|both}

  Specifies the phase to be run. Allowed values are one, two, or both. This argument is optional. The default is both.

  Typically you would run both phases. If a problem occurs during the second phase, then you can rerun phase two using the generated configuration file.

• dgname=diskgroup

  Specifies the name of the disk group to be renamed.

• newdgname=newdiskgroup

  Specifies the new name for the disk group.

• config=configfile

  Specifies the path to the configuration file to be generated during phase one or specifies the path to the configuration file to be used during phase two.

  This argument is optional. The default configuration file is named renamedg_config and is located in the directory in which the command is run. The single quotations may be required on some platforms.

• asm_diskstring=discoverystring, discoverystring ...

  Specifies the Oracle ASM discovery strings. The asm_diskstring value must be specified if the Oracle ASM disks are not in the default location for the platform. The single quotations may be required on some platforms, usually when wildcard characters are specified.

• clean={true|false}

  Specifies whether to clean errors that are otherwise ignored. The default is true.

• check={true|false}

  Specifies a boolean value that is used in the second phase. If true, then the tool prints the list of changes that are to be made to the disks. No writes are issued. It is an optional parameter that defaults to false.

• confirm={true|false}

  Specifies a boolean value that is used in the second phase. If false, then the tool prints the changes that are to be made and seeks confirmation before actually making the changes. It is an optional value that defaults to false. If check is set to true, then the value of this parameter is redundant.

• verbose={true|false}

  Specifies verbose execution when verbose=true. The default is false.

• keep_voting_files={true|false}

  Specifies whether voting files are kept in the renamed disk group. The default is false which deletes the voting files from the renamed disk group.

Example 4-12 shows several examples of the use of renamedg. The first example renames the fra1 disk group to fra2 using a disk string to locate the disks and the verbose option is enabled. The second example only creates a configuration file during the completion of phase one of the renamedg operation. The third example runs phase two of the renamedg operation using a configuration file generated from a phase one execution of renamedg.

After renaming a disk group, you can rename the disks in the disk group to match the new disk group name. For example:

SQL> ALTER DISKGROUP fra2 RENAME DISKS ALL;

For information about renaming disks, refer to Renaming Disks in Disk Groups.

$ renamedg dgname=fra1 newdgname=fra2 asm_diskstring='/devices/disk*'
      verbose=true

$ renamedg phase=one dgname=fra1 newdgname=fra2
      asm_diskstring='/devices/disk*' config=/tmp/fra2.conf verbose=true

$ renamedg phase=two dgname=fra1 newdgname=fra2 config=/tmp/fra2.conf
      verbose=true