Initialization Parameters Required for Oracle ASM (Automatic Storage Management)

The below parameters are required for ASM instance.

1. INSTANCE_TYPE 

INSTANCE_TYPE specifies whether the instance is a database instance or an Automatic Storage Management instance.

Values:

RDBMS

The instance is a database instance.

ASM

The instance is an Automatic Storage Management instance.


2. DB_UNIQUE_NAME 

DB_UNIQUE_NAME specifies a globally unique name for the database

Default value
Database instances: the value of DB_NAME
Automatic Storage Management instances: +ASM

3. ASM_POWER_LIMIT 

ASM_POWER_LIMIT specifies the maximum power on an Automatic Storage Management instance for disk rebalancing. The higher the limit, the faster rebalancing will complete. Lower values will take longer, but consume fewer processing and I/O resources.

Default value 1

Range of Values 0 to 11 (Prior to 11gR2)

Range of Values 0 to 1024(From 11gR2)

4. ASM_DISKGROUPS 

ASM_DISKGROUPS specifies a list of names of disk groups to be mounted by an Automatic Storage Management instance at instance startup or when an ALTER DISKGROUP ALL MOUNT statement is issued.

Automatic Storage Management (ASM) automatically adds a disk group to this parameter when the disk group is successfully created or mounted, and automatically removes a disk group from this parameter when the disk group is dropped or dismounted.

Issuing the ALTER DISKGROUP...ALL MOUNT or ALTER DISKGROUP...ALL DISMOUNT command does not affect the value of this parameter.

5.ASM_DISKSTRING 

ASM_DISKSTRING specifies an operating system-dependent value used by Automatic Storage Management to limit the set of disks considered for discovery. When a new disk is added to a disk group, each Automatic Storage Management instance that has the disk group mounted must be able to discover the new disk using the value of ASM_DISKSTRING.

6. ASM_PREFERRED_READ_FAILURE_GROUPS 

ASM_PREFERRED_READ_FAILURE_GROUPS specifies the failure groups that contain preferred read disks. Preferred disks are instance specific. This parameter is only valid in ASM instances.

What are Nodeapps Services in Oracle RAC

Nodeapps are standard set of oracle application services which are started automatically for RAC.

Node apps Include:

1) VIP.
2) Oracle Net listener.
3) Global Service Daemon.
4) Oracle Notification Service.

Nodeapp Services run on each node of the cluster and will switched over to other nodes through VIP during the failover.


Useful commands to maintain nodeapps services:

srvctl stop nodeapps -n NODE1 
[ STOP NODEAPPS  on NODE 1 ]
srvctl stop nodeapps -n NODE2            
[ STOP NODEAPPS  on NODE 2 ]

srvctl start nodeapps -n NODE1              
[ START NODEAPPS on NODE1  ]
srvctl start nodeapps -n NODE2             
[  START NODEAPPS ON NODE2 ]

srvctl status nodeapps                      
[Check the status of services on all nodes]

SQL Profile and SQL Plan Baseline

What is SQL Profile?

A SQL profile contains corrections for poor optimizer estimates discovered by the SQL Tuning advisor.A  SQL profile is to a SQL statement what statistics are to a table or index.
Implementing sql profiles allows us to replace bad sql execution plan with good execution plan.



What is SQL Plan Baseline?

A SQL plan baseline for a SQL statement consists of a set of accepted plans. When the statement is parsed, the optimizer will only select the best plan from among this set. If a different plan is found using the normal cost-based selection process, the optimizer will add it to the plan history but this plan will not be used until it is verified to perform better than the existing accepted plan and is evolved.

Oracle Database Administrator Interview Questions on RAC

Oracle RAC (Voting Disk ) FAQ's

1.What is Voting Disk?

Voting Disk is a file that maintains node membership details.
All members in the Cluster read and write Heartbeat Information in Voting Disk.

2. What Information is stored in Voting Disk?

Voting disks contain static and dynamic data.
Static data : Info about nodes in the cluster
Dynamic data : Disk heartbeat logging
It maintains and consists of important details about the cluster nodes membership, such as
- which node is part of the cluster,
- which node is joining the cluster, and
- which node is leaving the cluster.

3. Why do we need Voting Disk?

CSSD processes (Cluster Services Synchronization Daemon) monitor the health of  RAC nodes employing two distinct heart 

beats: Network heart beat and Disk heart beat. Healthy nodes will have continuous network and disk heartbeats exchanged 

between the  nodes. Break in heart beat indicates a possible error scenario. There are few different scenarios possible 

with missing heart beats:
1. Network heart beat is successful, but disk heart beat is missed.
2. Disk heart beat is successful, but network heart beat is missed.
3. Both heart beats failed.
In addition, with numerous nodes, there are other possible scenarios too. Few possible scenarios:
1. Nodes have split in to N sets of nodes, communicating within the set, but not with members in other set.
2. Just one node is unhealthy.
Nodes with quorum will maintain active membership of the cluster and other node(s) will be fenced/rebooted.

4.Why do we have odd number of voting disks?

Odd number of Voting disks are required to prevent Split Brain Syndrome.
A node must be able to access more than half of the voting disks in order to decide which node can be evicted incase of 

failure.

5. How to backup Voting Disk?

Prior to Oracle 11g R2 version, Voting Disk is backed up using dd command

From 11g R2, no need to manually backup voting disk.
It is automatically backup along with OCR  whenever there is a configuration change.

In 11g R2, Restoring Voting disk from a manually copied backupfile may prevent cluster services from starting up.