You can create an Oracle ASM Dynamic Volume Manager (Oracle ADVM) volume in a disk group.

The volume device associated with the dynamic volume can be used to host an Oracle ACFS file system.

The compatibility attributes COMPATIBLE.ASM and COMPATIBLE.ADVM must be set to 11.2 or higher for the disk group.

The ALTER DISKGROUP VOLUME SQL statements enable you to manage Oracle ADVM volumes, including the functionality to add, modify, resize, disable, enable, and drop volumes.

If the Oracle ADVM volume is on a flex disk group, then you can change the redundancy of the Oracle ADVM volume online with the ALTER DISKGROUP MODIFY FILEGROUP volume_name SQL statement.

The following are examples of the ALTER DISKGROUP VOLUME statement.

SQL> ALTER DISKGROUP data ADD VOLUME volume1 SIZE 10G;
Diskgroup altered.

SQL> ALTER DISKGROUP data RESIZE VOLUME volume1 SIZE 15G;
Diskgroup altered.

SQL> ALTER DISKGROUP data DISABLE VOLUME volume1;
Diskgroup altered.

SQL> ALTER DISKGROUP data ENABLE VOLUME volume1;
Diskgroup altered.

SQL> ALTER DISKGROUP ALL DISABLE VOLUME ALL;
Diskgroup altered.

SQL> ALTER DISKGROUP data DROP VOLUME volume1;
Diskgroup altered.

You can use the ADD clause of the ALTER DISKGROUP statement to add a disk or a failure group to a disk group.

You can add a disk or failure group with the ALTER DISKGROUP SQL statement. The syntax used is similar to the CREATE DISKGROUP statement. After you add new disks, the new disks gradually begin to accommodate their share of the workload as rebalancing progresses.

When adding a disk, the disk must be the same size as the other disks in the disk group.

A disk or multiple disks in a disk group can be replaced, rather than dropped and added back.

The single replace operation is more efficient than dropping and adding disks. This operation is especially useful when disks are missing or damaged.

For example, you can issue the following statement to replace the diskc7 disk with another disk identified by the /devices/diskc18 path.

SQL> ALTER DISKGROUP data2 REPLACE DISK diskc7 WITH '/devices/diskc18' POWER 3;

The power option operates the same as the power option for the ALTER DISKGROUP REBALANCE statement, except that the power option cannot be set to 0.

The ALTER DISKGROUP SQL statement with the REPLACE clause includes a WAIT or NOWAIT option, plus the FORCE option.

You can rename a disk in a disk group with the ALTER DISKGROUP RENAME DISK SQL statement.

In one SQL statement, you can rename one or multiple disks, or rename all disks in a disk group using the RENAME DISKS ALL clause.

For example, you can rename disks as follows:

SQL> ALTER DISKGROUP fra2 MOUNT RESTRICTED;

SQL> ALTER DISKGROUP fra2 RENAME DISK 'FRA1_0001' TO 'FRA2_0001', 
       'FRA1_0002' TO 'FRA2_0002';

For NORMAL, HIGH, and FLEX redundancy disk groups, you can associate a site name using ALTER DISKGROUP RENAME DISK with the SITE clause. For example:

SQL> ALTER DISKGROUP data1 RENAME DISK 'DATA1_0001' SITE SITE1;

For information about data sites, extended disk groups, and preferred read for disks, refer to About Oracle ASM Extended Disk Groups and PREFERRED_READ.ENABLED. For information about the ASMCMD stamp command, refer to stamp.

For an EXTERNAL redundancy disk group, you can use ALTER DISKGROUP RENAME DISK with the FAILGROUP clause to specify a failure group for a disk. For example:

SQL> ALTER DISKGROUP external1 RENAME DISK 'EXTERNAL1_0001' FAILGROUP FG_EXT1_0001;

The SITE and FAILGROUP clauses cannot be used together in the same SQL ALTER DISKGROUP RENAME statement.

The ALTER DISKGROUP diskgroupname RENAME DISKS ALL statement can be run after the renamedg utility to change the names of the disks in the renamed disk group. For information about renamedg, refer to Renaming Disks Groups.

When you run the ALTER DISKGROUP diskgroupname RENAME DISKS ALL statement, any disk name that is not in the format diskgroupname_number is renamed to that format. Disk names that are already in the diskgroupname_number format are not changed.

You can drop disks from a disk group with the DROP DISK clause of the ALTER DISKGROUP statement.

You can also drop all of the disks in specified failure groups using the DROP DISKS IN FAILGROUP clause.

When a disk is dropped, the disk group is rebalanced by moving all of the file extents from the dropped disk to other disks in the disk group. A drop disk operation might fail if not enough space is available on the other disks. The best approach is to perform both the add and drop operation with the same ALTER DISKGROUP statement. This has the benefit of rebalancing data extents only one time and ensuring that there is enough space for the rebalance operation to succeed.

If you specify the FORCE clause for the drop operation, the disk is dropped even if Oracle ASM cannot read or write to the disk. You cannot use the FORCE flag when dropping a disk from an external redundancy disk group.

The statements in Example 4-6 demonstrate how to drop disks from the disk group data1. The first example drops diska5 from disk group data1. The second example drops diska5 from disk group data1, and also illustrates how multiple actions are possible with one ALTER DISKGROUP statement.

ALTER DISKGROUP data1 DROP DISK diska5;

ALTER DISKGROUP data1 DROP DISK diska5
     ADD FAILGROUP failgrp1 DISK '/devices/diska9' NAME diska9;

The RESIZE clause of ALTER DISKGROUP enables you to resize disks in a disk group.

When resizing disks in a disk group, all the disks must be of equal size.

If you do not specify a new size in the SIZE clause, then Oracle ASM uses the size of the disks as returned by the operating system. The new size is written to the Oracle ASM disk header and if the size is increasing, then the new space is immediately available for allocation. If the size is decreasing, rebalancing must relocate file extents beyond the new size limit to available space below the limit. If the rebalance operation can successfully relocate all extents, then the new size is made permanent, otherwise the rebalance fails.

The following example resizes all of the disks in the disk group data1. If the new size is greater than the capacity of a disk, then the statement fails.

ALTER DISKGROUP data1 RESIZE ALL SIZE 100G;

The UNDROP DISKS clause of the ALTER DISKGROUP statement enables you to cancel all pending drops of disks within disk groups.

If a drop disk operation has completed, then this statement cannot be used to restore it. This statement cannot be used to restore disks that are being dropped as the result of a DROP DISKGROUP statement, or for disks that are being dropped using the FORCE clause.

The following example cancels the dropping of disks from disk group data1:

ALTER DISKGROUP data1 UNDROP DISKS;

You can manually rebalance the files in a disk group using the REBALANCE clause of the ALTER DISKGROUP statement.

Manual rebalancing would normally not be required because Oracle ASM automatically rebalances disk groups when their configuration changes, including changes to file groups. However, you might want to do a manual rebalance operation to control the speed of what would otherwise be an automatic rebalance operation.

The POWER clause of the ALTER DISKGROUP REBALANCE statement specifies the degree of parallelism, and the speed of the rebalance operation. A higher value increases the speed of the rebalance operation. It can be set to a minimum value of 0 which stops a rebalancing operation until the statement is either implicitly or explicitly rerun.

The default rebalance power is set by the ASM_POWER_LIMIT initialization parameter. The range of values for the POWER clause is the same for the ASM_POWER_LIMIT initialization parameter.

The power level of an ongoing rebalance operation can be changed by entering the rebalance statement using the MODIFY POWER clause. For example, the following SQL statement modifies the power setting to 10 of an ongoing rebalance.

ALTER DISKGROUP data2 REBALANCE MODIFY POWER 10;

The the following SQL statement modifies the power setting to the default value.

ALTER DISKGROUP data2 REBALANCE MODIFY POWER;

When you rebalance a disk group, you have the option choosing the following phase options using the WITH or WITHOUT keywords, except where noted:

• RESTORE

  The RESTORE phase includes RESYNC, RESILVER, and REBUILD operations. The RESTORE phase is always run by default and cannot be excluded.

  • RESYNC

    This operation synchronizes the stale extents on the disks that are being brought online.

  • RESILVER

    This operation is specific only to Exadata systems. During this phase, data is copied from one mirror to the mirror with stale data.

  • REBUILD

    This operation restores the redundancy of forcing disks only. Forcing disks are those disks that have been dropped with the force option.

• BALANCE

  This phase restores redundancy of all the disks in the disk group, including file groups, and also balances extents on Oracle ASM disks.

• PREPARE

  This phase completes the work corresponding to the prepare SQL operation. This phase is enabled only for FLEX or EXTENDED redundancy disk groups. COMPATIBLE.ASM must be set to 12.2 or higher.

• COMPACT

  This phase defragments and compacts extents across Oracle ASM disks.

The rebalance operation executes the phases in the optimal order. If none of the phases are specified with the ALTER DISKGROUP...REBALANCE SQL statement, then the behavior remains the same as the rebalance operation previous to the Oracle ASM 12.2 release during which all rebalance phases are run.

The following examples show the use of the phase options with ALTER DISKGROUP...REBALANCE:

ALTER DISKGROUP data2 REBALANCE WITH BALANCE COMPACT;

ALTER DISKGROUP data3 REBALANCE WITHOUT BALANCE;

The ALTER DISKGROUP...REBALANCE command returns immediately by default so that you can run other commands while the rebalance operation takes place asynchronously in the background. You can query the V$ASM_OPERATION view for the status of the rebalance operation. When not all rebalance phases are run, the V$ASM_OPERATION view displays an ORA-59000 error at the end of the rebalance.

To cause the ALTER DISKGROUP...REBALANCE command to wait until the rebalance operation is complete before returning, add the WAIT keyword to the REBALANCE clause. The wait functionality is especially useful in scripts. The command also accepts a NOWAIT keyword, which invokes the default behavior of conducting the rebalance operation asynchronously. You can interrupt a rebalance running in wait mode by typing CTRL+C on most platforms. This causes the command to return immediately with the message ORA-01013: user requested cancel of current operation, and then continues the rebalance operation asynchronously.

Additional rules for the rebalance operation include the following:

• An ongoing rebalance command is restarted if the storage configuration changes either when you alter the configuration, or if the configuration changes due to a failure or an outage. Furthermore, if the new rebalance fails because of a user error, then a manual rebalance may be required.

• The ALTER DISKGROUP...REBALANCE statement runs on a single node even if you are using Oracle Real Application Clusters (Oracle RAC).

• Oracle ASM can perform one disk group rebalance at a time on a given instance. If you have initiated multiple rebalances on different disk groups on a single node, then Oracle processes these operations in parallel on additional nodes if available; otherwise the rebalances are performed serially on the single node. You can explicitly initiate rebalances on different disk groups on different nodes in parallel.

• Rebalancing continues across a failure of the Oracle ASM instance performing the rebalance.

• The REBALANCE clause (with its associated POWER and WAIT/NOWAIT keywords) can also be used in ALTER DISKGROUP commands that add, drop, or resize disks.

  Note:

  Oracle restarts the processing of an ongoing rebalance operation if the storage configuration changes. If the next rebalance operation fails because of a user error, then a manual rebalance may be required.

The following example manually rebalances the disk group data2. The command does not return until the rebalance operation is complete. All supported phases of the rebalance operation are executed. Note that specifying RESTORE is not necessary as that phase is always run during a rebalance.

ALTER DISKGROUP data2 REBALANCE RESTORE POWER 5 WAIT;

If the POWER clause is not specified in an ALTER DISKGROUP statement, or when rebalance is implicitly run by adding or dropping a disk, then the rebalance power defaults to the value of the ASM_POWER_LIMIT initialization parameter. You can adjust the value of this parameter dynamically. The range of values for the POWER clause is the same for the ASM_POWER_LIMIT initialization parameter.

The higher the power limit, the more quickly a rebalance operation can complete. Rebalancing takes longer with lower power values, but consumes fewer processing and I/O resources which are shared by other applications, such as the database.

Oracle ASM tries to keep a rebalance I/O for each unit of power. Each I/O requires PGA memory for the extent involved in the relocation.

The default value of 1 minimizes disruption to other applications. The appropriate value is dependent on your hardware configuration, performance requirements, and availability requirements. Oracle ASM always adjusts the power to fit available memory.

When the COMPATIBLE.ASM disk group is set to 11.2.0.2 or higher, the rebalance operation may be run as one process using asynchronous I/O. You can check the Oracle ASM alert log for details on the rebalance process.

If a rebalance is in progress because a disk is manually or automatically dropped, then increasing the power of the rebalance shortens the time frame during which redundant copies of that data on the dropped disk are reconstructed on other disks.

You can also affect rebalance behavior with the CONTENT.CHECK and THIN_PROVISIONED disk group attributes.

The EXPLAIN WORK SQL statement determines the amount of work for a rebalance operation and the resulting calculations are displayed in the V$ASM_ESTIMATE view.

For example:

EXPLAIN WORK FOR ALTER DISKGROUP data DROP DISK data_0000;
Explained.

SELECT est_work FROM V$ASM_ESTIMATE;
EST_WORK
--------
    4211

EXPLAIN WORK SET STATEMENT_ID='online' FOR ALTER DISKGROUP data 
  ONLINE disk data_000;
Explained.

SELECT est_work FROM V$ASM_ESTIMATE WHERE STATEMENT_ID='online';
EST_WORK
--------
     421

The V$ASM_ESTIMATE view provides information for adjusting ASM_POWER_LIMIT and the resulting power of rebalance operations. The EST_WORK column provides an estimate of the number of allocation units that have to be moved by the rebalance operation to complete.

The PASS column of V$ASM_OPERATION is updated for resync and rebalance operations. The contents of the column can be RESYNC, REBALANCE, or COMPACT. For example, the following SQL query shows values in the PASS column during a rebalance operation.

SQL> SELECT GROUP_NUMBER, PASS, STATE FROM V$ASM_OPERATION;

GROUP_NUMBER PASS      STAT
------------ --------- ----
           2 RESYNC    WAIT
           2 REBALANCE WAIT
           2 COMPACT   WAIT

Oracle ASM disk scrubbing improves availability and reliability by searching for data that may be less likely to be read. Disk scrubbing checks logical data corruptions and repairs them automatically in normal and high redundancy disks groups. The scrubbing process repairs logical corruptions using the mirror disks. Disk scrubbing can be combined with disk group rebalancing to reduce I/O resources. The disk scrubbing process has minimal impact to the regular I/O in production systems.

You can perform scrubbing on a disk group, a specified disk, or a specified file of a disk group with the ALTER DISKGROUP SQL statement. For example, the following SQL statements show various options used when running the ALTER DISKGROUP disk_group SCRUB SQL statement.

SQL> ALTER DISKGROUP data SCRUB POWER LOW;

SQL> ALTER DISKGROUP data SCRUB FILE '+DATA/ORCL/DATAFILE/example.266.806582193' 
       REPAIR POWER HIGH FORCE;

SQL> ALTER DISKGROUP data SCRUB DISK DATA_0005 REPAIR POWER HIGH FORCE;

You can stop an ongoing scrub operation with the STOP option.

SQL> ALTER DISKGROUP data SCRUB STOP;

When using ALTER DISKGROUP with the SCRUB option, the following items apply:

• The optional REPAIR option automatically repairs disk corruptions. If the REPAIR option is not specified, then the SCRUB option only checks and reports logical corruptions of the specified target.

• The optional POWER value can be set to AUTO, LOW, HIGH, or MAX. If the POWER option is not specified, the power value defaults to AUTO and the power adjusts to the optimum level for the system.

• If the optional WAIT option is specified, the command returns after the scrubbing operation has completed. If the WAIT option is not specified, the scrubbing operation is added into the scrubbing queue and the command returns immediately.

• If the optional FORCE option is specified, the command is processed even if the system I/O load is high or scrubbing has been disabled internally at the system level.

The scrubbing process is visible in the fields of the V$ASM_OPERATION view while the scrubbing process is running.