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October 20, 2015

Oracle ASM Interview Questions/FAQs

Oracle ASM/DBA Interview Questions


1. What is the use of ASM (or) Why ASM preferred over filesystem?
ASM provides striping and mirroring.
a) Provides automatic load balancing over all the available disks, thus reducing hot spots in the file system.
b) Prevents fragmentation of disks, so you don’t need to manually relocate data to tune I/O performance.
c) Adding disks is straight forward – ASM automatically performs online disk reorganization when you add or remove storage.
d) Uses redundancy features available in intelligent storage arrays.
e)The storage system can store all types of database files.
f) Using disk group makes configuration easier, as files are placed into disk groups.

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. When you will use external redundancy and what are the advantages of it?

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

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

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

9. What happen if you miss "+" sign while adding datafile in ASM disk group ?

10. 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;

11. 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, a slave for rebalancing operations.

12. What processes does the rebalancing?
RBAL, ARBx

13. How to dismount ASM disk group from particular instance in RAC?

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

Related Articles: Oracle Data Guard Interview Questions/FAQs  Oracle DBA Interview Questions/FAQs – Part2  UNIX Interview Questions/FAQs for Oracle DBAs

1 comment:

  1. Hello There,


    In debt to you for making my learning on the Oracle ASM Interview Questions/FAQs area so hassle-free! I lay my faith on your writings.

    I am facing the below issue(Screen shot) with CREATE_JOB procedure. The issue is with CREDENTIAL_NAME attribute.

    More information on this:

    I tried to create the job for one of the predefined chain
    why I am using CREDENTIAL_NAME attribute is, in one of the chain step I am calling LINUX script.

    I read multiple articles and watched many videos about how to use this tool - and was still confused! Your instructions were easy to understand and made the process simple.


    Many Thanks,
    Uday

    ReplyDelete