Oracle RMAN (Recovery Manager) Concepts

What is the difference between obsolete and expired Backup in RMAN?

Obsolete Backup: Backup pieces or backup sets which are not needed for recovery.

We can use the REPORT OBSOLETE command to list the obsolete backups and DELETE OBSOLETE command to delete the obsolete backup.

Expired Backup:When the CROSSCHECK command is used to determine whether backups recorded in the repository still exist on disk or tape, if RMAN cannot locate the backups, then it updates their records in the RMAN repository to EXPIRED status.We can then use the DELETE EXPIRED command to remove records of expired backups from the RMAN repository.

Oracle DBA Concepts: Background Processes in Oracle ASM (Automatic storage Management)

The following background processes are an integral part of Automatic Storage Management:

ARBn performs the actual rebalance data extent movements in an Automatic Storage Management instance. There can be many of these processes running at a time, named ARB0, ARB1, and so on.

ASMB runs in a database instance that is using an ASM disk group. ASMB communicates with the ASM instance, managing storage and providing statistics. ASMB can also run in the ASM instance. ASMB runs in ASM instances when the ASMCMD cp command runs or when the database instance first starts if the SPFILE is stored in ASM.

GMON maintains disk membership in ASM disk groups.

MARK marks ASM allocation units as stale following a missed write to an offline disk. This essentially tracks which extents require resync for offline disks.

RBAL runs in both database and ASM instances. In the database instance, it does a global open of ASM disks. In an ASM instance, it also coordinates rebalance activity for disk groups.

How to change Rebalance Power in ASM Instances?

Whenever a new diskgroup is added or dropped, ASM automatically performs REBALANCING operation.Power used by rebalacing defaults to value specified for asm_power_limit initialization parameter.We can check the status of rebalancing from v$asm_operation view.

Increasing the value of ASM_POWER_LIMIT, reduces the estimated time for completion of Rebalance Operation.

ASM power limit can be increased using the below command

alter system set asm_power_limit =11;

To know more about values for this parameter, check this link:

http://www.appsdbadiaries.com/2017/06/asm-features-rebalancing.html

But this does not increase the rebalancing power of current operation. 

To increase the rebalancing power of ongoing operation, use the below command

 alter diskgroup dg1 rebalance power 11;

We can also specify rebalance power at the time of adding or dropping disks.

alter diskgroup dg1 add disk d01 rebalance power 11;

When to use asm_power_limit 0?

Whenever there is a need to add/drop several disks (like migrations), setting the power limit to 0 is highly beneficial.We can avoid the waiting time for rebalance operation to complete,before the next disk is rebalanced.In such cases, set the asm_power_limit to 0, add/drop all the disks and set the value of asm_power_limit to a non zero value.

Rebalance operation will be parallelized and completes faster.