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October 20, 2012

Oracle ASM Interview Questions/FAQs

Oracle ASM Interview Questions/FAQs


1. What is the use of ASM (or) Why ASM preferred over filesystem?
ASM provides striping and mirroring.

2. What are the init parameters related to ASM?
INSTANCE_TYPE = ASM
ASM_POWER_LIMIT = 11
ASM_DISKSTRING = '/dev/rdsk/*s2', '/dev/rdsk/c1*'
ASM_DISKGROUPS = DG_DATA, DG_FRA

3. What is rebalancing (or) what is the use of ASM_POWER_LIMIT?
ASM_POWER_LIMIT is dynamic parameter, which will be useful for rebalancing the data across disks.
Value can be 1(lowest) to 11 (highest).

4. What are different types of redundancies in ASM & explain?
  • Normal redundancy - for 2-way mirroring, requiring two failure groups, when ASM allocates an extent for a normal redundancy file, ASM allocates a primary copy and a secondary copy. ASM chooses the disk on which to store the secondary copy in a different failure group other than the primary copy.
  • High redundancy - for 3-way mirroring, requiring three failure groups, in this case the extent is mirrored across 3 disks.
  • External redundancy - to not use ASM mirroring. This is used if you are using hardware mirroring or third party redundancy mechanism like RAID, Storage arrays.

5. How to copy file to/from ASM from/to filesystem?
By using ASMCMD cp command

6. How to find out the databases, which are using the ASM instance?
ASMCMD> lsct
SQL> select DB_NAME from V$ASM_CLIENT;

7. What are different types of stripings in ASM & their differences?
Fine-grained striping
Coarse-grained striping

8. What is allocation unit and what is default value of au_size and how to change?
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';
lsdg
select NAME,ALLOCATION_UNIT_SIZE from v$asm_diskgroup;

9. What are the background processes in ASM?
  • ASMB - This ASMB process is used to provide information to and from cluster synchronization services used by ASM to manage the disk resources. It's also used to update statistics and provide a heart beat mechanism.
  • RBAL, Re-Balance - RBAL is the ASM related process that performs rebalancing of disk resources controlled by ASM.
  • ARBx, Actual Rebalance - ARBx is configured by ASM_POWER_LIMIT.

10. What processes does the rebalancing?
RBAL, ARBn

11. How to add/remove disk to/from diskgroup?
ALTER DISKGROUP data1 ADD DISK '/devices/diska5';
ALTER DISKGROUP disk_group_1 DROP DISK diska2;

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