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?
SQL> select DB_NAME from V$ASM_CLIENT;
7. What are different types of stripings in ASM & their differences?
8. What is allocation unit and what is default value of au_size and how to change?
Every ASM disk is divided into allocation units (AU). An AU is the fundamental unit of allocation within a disk group. A file extent consists of one or more AU. An ASM file consists of one or more file extents.
CREATE DISKGROUP disk_group_2 EXTERNAL REDUNDANCY DISK '/dev/sde1' ATRRIBUTE 'au_size' = '32M';
select NAME,ALLOCATION_UNIT_SIZE from v$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?
11. How to add/remove disk to/from diskgroup?