ADDITIONS/ENHANCEMENTS IN AUTOMATIC STORAGE MANAGEMENT (ASM)

ADDITIONS/ENHANCEMENTS IN AUTOMATIC STORAGE MANAGEMENT (ASM)

Flex ASM
In a typical Grid Infrastructure installation, each node will have its own ASM instance running and act the as the storage container for the databases running on the node. There is a single point-of-failure threat with this setup. For instance, if the ASM instance on the node suffers or fails all the databases and instances running on the node will be impacted. To avoid ASM instance single-point-failure, Oracle 12c provides a Flex ASM feature. The Flex ASM is a different concept and architecture all together. Only a fewer number of ASM Instances need to run on a group of servers in the cluster. When an ASM instance fails on a node, Oracle Clusterware automatically starts surviving (replacement) ASM instance on a different node to maintain availability. In addition, this setup also provides ASM instance load balancing capabilities for the instances running on the node. Another advantage of Flex ASM is that it can be configured on a separate node.

When you choose Flex Cluster option as part of the cluster installation, Flex ASM configuration will be automatically selected as it is required by the Flex Cluster. You can also have traditional cluster over Flex ASM. When you decide to use Flex ASM, you must ensure the required networks are available.  You can choose the Flex ASM storage option as part of Cluster installation, or use ASMCA to enable Flex ASM in a standard cluster environment.

The following command shows the current ASM mode:

$ ./asmcmd showclustermode

$ ./srvctl config asm

Or connect to the ASM instances and query the INSTANCE_TYPE parameter. If the output value is ASMPROX, then, the Flex ASM is configured.

Increased ASM storage limits

The ASM storage hard limits on maximum ASM disk groups and disk size has been drastically increased. In 12cR1, ASM support 511 ASM disk groups against 63 ASM disk groups in 11gR2. Also, an ASM disk can be now 32PB size against 20PB in 11gR2.

Tuning ASM rebalance operations

The new EXPLAIN WORK FOR statement in 12c measures the amount of work required for a given ASM rebalance operation and inputs the result in V$ASM_ESTIMATE dynamic view. Using the dynamic view, you can adjust thePOWER LIMIT clause to improve the rebalancing operation work. For example, if you want to measure the amount of work required for adding a new ASM disk, before actually running the manual rebalance operation, you can use the following:

SQL> EXPLAIN WORK FOR ALTER DISKGROUP DG_DATA ADD DISK data_005;

SQL> SELECT est_work FROM V$ASM_ESTIMATE;

' FOR ALTER DISKGROUP DG_DATA AD DISK data_005;

SQL> SELECT est_work FROM V$ASM_ESTIMATE W

SQL> EXPLAIN WORK SET STATEMENT_ID='ADD_DISK HERE STATEMENT_ID = 'ADD_DISK’;

You can adjust the POWER limit based on the output you get from the dynamic view to improve the rebalancing operations.

ASM Disk Scrubbing

The new ASM Disk Scrubbing operation on a ASM diskgroup with normal or high redundancy level, verifies the logical data corruption on all ASM disks of that ASM diskgroup, and repairs the logical corruption automatically, if detected, using the ASM mirror disks. The disk scrubbing can be performed at disk group, specified disk or on a file and the impact is very minimal. The following examples demonstrate the disk scrubbing scenario:

SQL> ALTER DISKGROUP dg_data SCRUB POWER LOW:HIGH:AUTO:MAX;

SQL> ALTER DISKGROUP dg_data SCRUB FILE '+DG_DATA/MYDB/DATAFILE/filename.xxxx.xxxx'REPAIR POWER AUTO;

Active Session History (ASH) for ASMThe V$ACTIVE_SESSION_HISOTRY dynamic view now provides the active session sampling on ASM instance too. However, the use of diagnostic pack is subject to the license.