ASM Interview questions

1) What are the background processes in ASM

Ans:
RABL- Rebalancer: It opens all the device files as part of disk discovery and coordinates the ARB processes for rebalance activity.

ARBx - Actual Rebalancer: They perform the actual rebalancing activities. 
The number of ARBx processes depends on the ASM_POWER_LIMIT init parameter.

ASMB - ASM Bridge: This process is used to provide information to and from the Cluster Synchronization Service (CSS) used by ASM to manage the disk resources. 
It is also used to update statistics and provide a heartbeat mechanism.


2) What is the use of ASM (or) Why ASM preferred over filesystem?

ANS: ASM provides striping and mirroring.


3) What are the init parameters related to ASM?

ANS:
INSTANCE_TYPE = ASM
ASM_POWER_LIMIT = 11
ASM_DISKSTRING = '/dev/rdsk/*s2', '/dev/rdsk/c1*'
ASM_DISKGROUPS = DG_DATA, DG_FRA


4) What is rebalancing (or) what is the use of ASM_POWER_LIMIT?

ANS:
ASM_POWER_LIMIT is dynamic parameter, which will be useful for rebalancing the data across disks.
Value can be 1(lowest) to 11 (highest).


5) What are different types of redundancies in ASM & explain?

ANS:
External redundancy,
Normal redundancy,
High redundancy.

6) How to copy file to/from ASM from/to filesystem?

ANS:
By using ASMCMD cp command

7) How to find out the databases, which are using the ASM instance?

ANS:

ASMCMD> lsct
DB_Name   Status     Software_Version  Compatible_version  Instance_Name  Disk_Group
amxdcmp1  CONNECTED        11.2.0.2.0          11.2.0.2.0  amxdcmp1       DG1_DCM_DATA
amxddip1  CONNECTED        11.2.0.2.0          11.2.0.2.0  amxddip1       DG1_DDI_DATA
ASMCMD>

(or)

SQL> select DB_NAME from V$ASM_CLIENT;

8) What are different types of stripings in ASM & their differences?

ANS:
Fine-grained striping
Coarse-grained striping

ASMCMD> lsdg
State    Type    Rebal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Voting_files  Name
MOUNTED  EXTERN  N         512   4096  1048576   6835200  1311391                0         1311391              0             N  DG1_DCM_DATA/
MOUNTED  EXTERN  N         512   4096  1048576    486400   154487                0          154487              0             N  DG1_DDI_DATA/
ASMCMD>


SQL> select NAME,ALLOCATION_UNIT_SIZE/1024/1024 "MB" from v$asm_diskgroup;

NAME                                   MB
------------------------------ ----------
DG1_DCM_DATA                            1
DG1_DDI_DATA                            1

9) What is allocation unit and what is default value of au_size and how to change?

ANS:
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';

10) What process does the rebalancing?

ANS:
RBAL, ARBn

11) How to add/remove disk to/from diskgroup?

ANS:
add disk:

ALTER DISKGROUP DG1_ZABBIX_DATA ADD DISK 
'/zabbix_u03/oradata/zbxprd1/ZBX_DATA_DISK009' name ZBX_DATA_DISK009,
'/zabbix_u04/oradata/zbxprd1/ZBX_DATA_DISK010' name ZBX_DATA_DISK010,
'/zabbix_u05/oradata/zbxprd1/ZBX_DATA_DISK011' name ZBX_DATA_DISK011;

remove disk:

alter diskgroup DG1_CIE_DATA drop disk
DG_CIE_DATA_DISK001, 
DG_CIE_DATA_DISK002, 
DG_CIE_DATA_DISK003, 
DG_CIE_DATA_DISK004;