What happen if you miss “+” sign while adding datafile in ASM disk group ?

Datafile gets created on Normal file system at dbs location.we need to later convert this datafile to normal file systems to ASM filesystem with RMAN.

What is allocation unit and what is default value of au_size?

Every ASM disk is divided into allocation units (AU). An AU is the fundamental unit of allocation within a disk group. A file extent consists of one or more AU. An ASM file consists of one or more file extents.

CREATE DISKGROUP disk_group_2 EXTERNAL REDUNDANCY DISK '/dev/sde1' ATRRIBUTE 'au_size' = '32M';
FINDING Allocation unit size FROM ASMCMD COMMAND:
amcmd > lsdg
FINDING Allocation unit size  FROM SQL 
 SQL >select NAME,ALLOCATION_UNIT_SIZE from v$asm_diskgroup;

What is rebalancing?When it happens?Which processes does rebalancing?Explain FLOW of ASM rebalancing?

Any change in the storage configuration like disk addition or dropping will trigger a oracle ASM rebalance operation  The main objective of the oracle ASM rebalance operation is to always provide an even distribution of file extents and space usage across all disks in the diskgroup. Rebalancing is performed on all database files on a per file basis. The Oracle background process, RBAL, from the ASM instance manages this rebalance.

Process flow for ASM rebalancing:
1. On the ASM instance, a disk is added or dropped from a  diskgroup.
2. This invokes the RBAL process to create the rebalance plan and then begin coordination of the
redistribution
3. RBAL will calculate estimation time and work required to perform the task and then message the
ARBx processes to actually handle the request. The number of ARBx processes invoked is
directly determined by the asm_power_limit.
4. The Continuing Operations Directory (metadata) will be updated to reflect a rebalance activity.
5. Each extent to be relocated is assigned to an ARBx process.
6. ARBx performs rebalance on these extents. Each extent is locked, relocated, and unlocked. This
is shown as Operation REBAL in V$ASM_OPERATION

The time of rebalance depends on the init.ora parameter asm_power_limit.The range of values for asm_power_limit are 0 to 11; where a value of 11 is full throttle and a value of 1 is low speed. A value of 0, which turns off automatic rebalance, should be used with caution.It is recommended that  If removing or adding several disks,  it is best to add or remove drives all at once, this will reduce the number rebalance operations that are needed for storage changes .

How to find ASM Rebalancing status from SQL?

Select * from v$asm_operation;

How to add/remove disk to/from diskgroup?

Check If disks are available with status CANDIDATE or PROVISIONED in V$ASM_DISK.If disk are available use below command to add into diskgroup.

ALTER DISKGROUP data1 ADD DISK '/devices/diska5';

If the disk are part of any diskgroup you can use below command to drop the disk from diskgroup.

ALTER DISKGROUP disk_group_1 DROP DISK diska2;

What is ASM Disks ?

The physical disks are known as ASM disks

What is ASM diskgroups?

ASM disk groups, each of which comprise of several physical disks that are controlled as a single unit

What is Failure groups

They are defined within a disk group to support the required level of redundancy. For two-way mirroring you would expect a disk group to contain two failure groups so individual files are written to two locations.

Why should we use separate ASM home?

ASM should be installed separately from the database software in its own ORACLE_HOME directory. This will allow you the flexibility to patch and upgrade ASM and the database software independently.

How many ASM instances should one have?

Several databases can share a single ASM instance. So, although one can create multiple ASM instances on a single system, normal configurations should have one and only one ASM instance per system.

For clustered systems, create one ASM instance per node (called +ASM1, +ASM2, etc).

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