Oracle DBA Interview Questions and Answers

Explain the difference between a hot backup and a cold backup.

A cold backup is done when there is no user activity going on with the system. Also called as offline backup, is taken when the database is not running and no users are logged in. all files of the database are copied and no changes during the copy are made.

A hot backup is taken when the database needs to run all the time. It is an online backup. All files of the database are copied and there may be changes to the database during the copy.

How many memory layers are in the oracle shared pool? Explain them

Oracles shared pool consists of two layers namely, Library cache and Data dictionary cache.

Library cache: This layer has information about SQL statements that were parsed, information about cursors and any plan data.

Data Dictionary cache: this Layer has information about the accounts of the users, their privileges and segments information.

What is the cache hit ratio, what impact does it have on performance of an Oracle database and what is involved in tuning it?

When the cache client like a CPU, web browser etc finds a successful entry in the cache memory, it is called as a cache hit. The percentage of these successful cache hits is called as cache hit ratio. Higher the cache hit ratio better will be the performance because data read from the cache is faster than the memory.

Explain the concept of the DUAL table.

On installing Oracle database, DUAL table Is present by default. It is a special table with just one row. It has a single column called DUMMY. The data type of this column in VARCHAR2(1). It has a value “X”. It is most commonly used to select pseudo columns in Oracle like sysdate.

Select sysdate from dual 

What are the ways tablespaces can be managed and how do they differ?

Objects can be assigned to a table space. The related objects can be then grouped together. Table space can also be managed using extents. Extents consist of a specific number of contiguous data blocks. For the required extent, the free extent closest in size is determined.

Explain what are Oracle Catalog and Oracle Archive log?

Oracle catalog contains tables and views to get information about the database. It helps user to understand the available tables, attributes, constraints etc.

Oracle Archive log mode of a database in Oracle, ensures, the online redo logs are not overwritten before they are archived. This ensures that recovery is possible.

What are PCT Free and PCT Used? What is PCT increase parameter in segment?

PCTFREE is a parameter used to find how much space should be left in a database block for future updates. This means that if the PCTFREE = 20, new rows will be added in the block until it is 80% full.

PCTUSED is a parameter helps Oracle to find when it should consider a database block to be empty enough to be added to the freelist. This means that if the PCTFREE = 50, new rows will be not be added in the block until sufficient rows are deleted from the block so that it falls below 40% empty.

PCTINCREASE parameter is used to find how much will the each subsequent segment will grow. This value is in %.

What is dump destination? What are bdump, cdump and udump?

Trace files for Oracle processes are stored in dump destination.

Bdump- Oracle writes to the trace log and creates trace files for background processes in background dump destination. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Cdump- Oracle writes core files and background processes in Core dump destination directory. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Udump – Oracle creates trace files for the user processes in the User Dump directory if this directory becomes full and more files cannot be written, debugging becomes difficult.

How do you increase the performance of %LIKE operator?

LIKE% works the fastest because it uses the index to search on the column provided an index is specified on the column. Using % after LIKE, results in faster results.

Why use materialized view instead of a table

Materialized views are basically used to increase query performance since it contains results of a query. They should be used for reporting instead of a table for a faster execution.

Why and how the deadlock situation arises

A deadlock situation arises when two or more users wait for the same resource locked by one anther or two or more processes wait to update rows which are locked by other processes. Oracle if detects a deadlock, rolls back the session chosen by the deadlock victim.

What are standby databases? Difference between Physical and logical standby databases

A standby database is a replica of the original database. In order to keep both the database synchronized, archived redo logs can be used. It is mainly used in disaster protection. It can also be opened in read only mode which allows it to be used independently for reporting.

A logical standby database allows new database objects like tables, indexes to be added to the database. On the other hand, the physical standby database is a physical or structural copy of primary database. They can be opened in read only for disaster recovery.

What is Cache Fusion Technology?

In Cache fusion, multiple buffers join to act as one. It eliminates disk i/o operaions by making use of a scalable shared cache. It treats multiple buffer caches as one thereby resolving data consistency issues. Cash fusion technology can provide more resources and increases concurrency of users.

What is the difference between Cloning and Standby databases?

The clone database is a copy of the database which can be opened in read write mode. It is treated as a separate copy of the database that is functionally completely separate. The standby database is a copy of the production database used for disaster protection. In order to update the standby database; archived redo logs from the production database can be used. If the primary database is destroyed or its data becomes corrupted, one can perform a failover to the standby database, in which case the standby database becomes the new primary database.

RAC QUESTIONS

1. What is the major difference between 10g and 11g RAC?

Well, there is not much difference between 10g and 11gR (1) RAC.

But there is a significant difference in 11gR2.

Prior to 11gR1(10g) RAC, the following were managed by Oracle CRS

• Databases
• Instances
• Applications
• Node Monitoring
• Event Services
• High Availability

From 11gR2(onwards) its completed HA stack managing and providing the following resources as like the other cluster software like VCS etc.

• Databases
• Instances
• Applications
• Cluster Management
• Node Management
• Event Services
• High Availability
• Network Management (provides DNS/GNS/MDNSD services on behalf of other traditional services) and SCAN – Single Access Client Naming method, HAIP
• Storage Management (with help of ASM and other new ACFS filesystem)
• Time synchronization (rather depending upon traditional NTP)
• Removed OS dependent hang checker etc, manages with own additional monitor process

2.  What are Oracle Cluster Components?

Cluster Interconnect (HAIP)

Shared Storage (OCR/Voting Disk)

Clusterware software

3. What are Oracle RAC Components?

VIP, Node apps etc.

4. What are Oracle Kernel Components (nothing but how does Oracle RAC database differs than Normal single instance database in terms of Binaries and process)

Basically Oracle kernel need to switched on with RAC On option when you convert to RAC, that is the difference as it facilitates few RAC bg process like LMON,LCK,LMD,LMS etc.

To turn on RAC
# link the oracle libraries
$ cd $ORACLE_HOME/rdbms/lib
$ make -f ins_rdbms.mk rac_on
# rebuild oracle
$ cd $ORACLE_HOME/bin
$ relink oracle

Oracle RAC is composed of two or more database instances. They are composed of Memory structures and background processes same as the single instance database.Oracle RAC instances use two processes GES(Global Enqueue Service), GCS(Global Cache Service) that enable cache fusion.Oracle RAC instances are composed of following background processes:

ACMS—Atomic Controlfile to Memory Service (ACMS)
GTX0-j—Global Transaction Process
LMON—Global Enqueue Service Monitor
LMD—Global Enqueue Service Daemon
LMS—Global Cache Service Process
LCK0—Instance Enqueue Process
RMSn—Oracle RAC Management Processes (RMSn)
RSMN—Remote Slave Monitor

5. What is Clusterware?

Software that provides various interfaces and services for a cluster. Typically, this includes capabilities that:

• Allow the cluster to be managed as a whole
• Protect the integrity of the cluster
• Maintain a registry of resources across the cluster
• Deal with changes to the cluster
• Provide a common view of resources

6. What are the background process that exists in 11gr2 and functionality?

Process Name	Functionality
crsd	•The CRS daemon (crsd) manages cluster resources based on configuration information that is stored in Oracle Cluster Registry (OCR) for each resource. This includes start, stop, monitor, and failover operations. The crsd process generates events when the status of a resource changes.
cssd	•Cluster Synchronization Service (CSS): Manages the cluster configuration by controlling which nodes are members of the cluster and by notifying members when a node joins or leaves the cluster. If you are using certified third-party clusterware, then CSS processes interfaces with your clusterware to manage node membership information. CSS has three separate processes: the CSS daemon (ocssd), the CSS Agent (cssdagent), and the CSS Monitor (cssdmonitor). The cssdagent process monitors the cluster and provides input/output fencing. This service formerly was provided by Oracle Process Monitor daemon (oprocd), also known as OraFenceService on Windows. A cssdagent failure results in Oracle Clusterware restarting the node.
diskmon	•Disk Monitor daemon (diskmon): Monitors and performs input/output fencing for Oracle Exadata Storage Server. As Exadata storage can be added to any Oracle RAC node at any point in time, the diskmon daemon is always started when ocssd is started.
evmd	•Event Manager (EVM): Is a background process that publishes Oracle Clusterware events
mdnsd	•Multicast domain name service (mDNS): Allows DNS requests. The mDNS process is a background process on Linux and UNIX, and a service on Windows.
gnsd	•Oracle Grid Naming Service (GNS): Is a gateway between the cluster mDNS and external DNS servers. The GNS process performs name resolution within the cluster.
ons	•Oracle Notification Service (ONS): Is a publish-and-subscribe service for communicating Fast Application Notification (FAN) events
oraagent	•oraagent: Extends clusterware to support Oracle-specific requirements and complex resources. It runs server callout scripts when FAN events occur. This process was known as RACG in Oracle Clusterware 11g Release 1 (11.1).
orarootagent	•Oracle root agent (orarootagent): Is a specialized oraagent process that helps CRSD manage resources owned by root, such as the network, and the Grid virtual IP address
oclskd	•Cluster kill daemon (oclskd): Handles instance/node evictions requests that have been escalated to CSS
gipcd	•Grid IPC daemon (gipcd): Is a helper daemon for the communications infrastructure
ctssd	•Cluster time synchronisation daemon(ctssd) to manage the time syncrhonization between nodes, rather depending on NTP

7. Under which user or owner the process will start?

Component	Name of the Process	Owner
Oracle High Availability Service	ohasd	init, root
Cluster Ready Service (CRS)	Cluster Ready Services	root
Cluster Synchronization Service (CSS)	ocssd,cssd monitor, cssdagent	grid owner
Event Manager (EVM)	evmd, evmlogger	grid owner
Cluster Time Synchronization Service (CTSS)	octssd	root
Oracle Notification Service (ONS)	ons, eons	grid owner
Oracle Agent	oragent	grid owner
Oracle Root Agent	orarootagent	root
Grid Naming Service (GNS)	gnsd	root
Grid Plug and Play (GPnP)	gpnpd	grid owner
Multicast domain name service (mDNS)	mdnsd	grid owner

8. As you said Voting & OCR Disk resides in ASM Diskgroups, but as per startup sequence OCSSD starts first before than ASM, how is it possible?

How does OCSSD starts if voting disk & OCR resides in ASM Diskgroups?

You might wonder how CSSD, which is required to start the clustered ASM instance, can be started if voting disks are stored in ASM? This sounds like a chicken-and-egg problem: without access to the voting disks there is no CSS, hence the node cannot join the cluster. But without being part of the cluster, CSSD cannot start the ASM instance. To solve this problem the ASM disk headers have new metadata in 11.2: you can use kfed to read the header of an ASM disk containing a voting disk. The kfdhdb.vfstart and kfdhdb.vfend fields tell CSS where to find the voting file. This does not require the ASM instance to be up. Once the voting disks are located, CSS can access them and joins the cluster.

Source: Pro Oracle Database 11g RAC on Linux- Martin Bach … – Amazon.com

9. How does SCAN works?

1. Client Connected through SCAN name of the cluster (remember all three IP addresses round robin resolves to same Host name (SCAN Name), here in this case our scan name is cluster01-scan.cluster01.example.com
2. The request reaches to DNS server in your corp and then resolves to one of the node out of three.  a. If GNS (Grid Naming service or domain is configured) that is a subdomain configured in  the DNS entry for to resolve cluster address the request will be handover to GNS (gnsd)
3. Here in our case assume there is no GNS, now the with the help of SCAN listeners where end points are configured to database listener.
4. Database Listeners listen the request and then process further.
5. In case of node addition, Listener 4, client need not to know or need not change any thing from their tns entry (address of 4^thnode/instance) as they just using scan IP.
6. Same case even in the node deletion.

10. What is GNS?

Grid Naming service is alternative service to DNS , which will act as a sub domain in your DNS but managed by Oracle, with GNS the connection is routed to the cluster IP and manages internally.

11. What is GPNP?

Grid Plug and Play along with GNS provide dynamic

In previous releases, adding or removing servers in a cluster required extensive manual preparation.

In Oracle Database 11g Release 2, GPnP allows each node to perform the following tasks dynamically:

• Negotiating appropriate network identities for itself
• Acquiring additional information from a configuration profile
• Configuring or reconfiguring itself using profile data, making host names and addresses resolvable on the network

For example a domain should contain

• –Cluster name: cluster01
• –Network domain: example.com
• –GPnP domain: cluster01.example.com

To add a node, simply connect the server to the cluster and allow the cluster to configure the node.

To make it happen, Oracle uses the profile located in $GI_HOME/gpnp/profiles/peer/profile.xml which contains the cluster resources, for example disk locations of ASM. etc.

So this profile will be read local or from the remote machine when plugged into cluster and dynamically added to cluster.

12. What are the file types that ASM support and keep in disk groups?

Control files	Flashback logs	Data Pump dump sets
Data files	DB SPFILE	Data Guard configuration
Temporary data files	RMAN backup sets	Change tracking bitmaps
Online redo logs	RMAN data file copies	OCR files
Archive logs	Transport data files	ASM SPFILE

13. List Key benefits of ASM?

• Stripes files rather than logical volumes
• Provides redundancy on a file basis
• Enables online disk reconfiguration and dynamic rebalancing
• Reduces the time significantly to resynchronize a transient failure by tracking changes while disk is offline
• Provides adjustable rebalancing speed
• Is cluster-aware
• Supports reading from mirrored copy instead of primary copy for extended clusters
• Is automatically installed as part of the Grid Infrastructure

14. List key benefits of Oracle Grid Infrastructure?

15. List some of the background process that used in ASM?

Process	Description
RBAL	Opens all device files as part of discovery and coordinates the rebalance activity
ARBn	One or more slave processes that do the rebalance activity
GMON	Responsible for managing the disk-level activities such as drop or offline and advancing the ASM disk group compatibility
MARK	Marks ASM allocation units as stale when needed
Onnn	One or more ASM slave processes forming a pool of connections to the ASM instance for exchanging messages
PZ9n	One or more parallel slave processes used in fetching data on clustered ASM installation from GV$ views

16. What is node listener?

In 11gr2 the listeners will run from Grid Infrastructure software home

• The node listener is a process that helps establish network connections from ASM clients to the ASM instance.
• Runs by default from the Grid $ORACLE_HOME/bin directory
• Listens on port 1521 by default
• Is the same as a database instance listener
• Is capable of listening for all database instances on the same machine in addition to the ASM instance
• Can run concurrently with separate database listeners or be replaced by a separate database listener
• Is named tnslsnr on the Linux platform

17. What is SCAN listener?

A scan listener is something that additional to node listener which listens the incoming db connection requests from the client which got through the scan IP, it got end points configured to node listener where it routes the db connection requests to particular node listener.

18. What is the difference between CRSCTL and SRVCTL?

crsctl manages clusterware-related operations:

• Starting and stopping Oracle Clusterware
• Enabling and disabling Oracle Clusterware daemons
• Registering cluster resources

srvctl manages Oracle resource–related operations:

• Starting and stopping database instances and services
• Also from 11gR2 manages the cluster resources like network,vip,disks etc

19. How to control Oracle Clusterware?

To start or stop Oracle Clusterware on a specific node:

# crsctl stop crs

# crsctl start crs

To enable or disable Oracle Clusterware on a specific node:

# crsctl enable crs

# crsctl disable crs

20. How to check the cluster (all nodes) status?

To check the viability of Cluster Synchronization Services (CSS) across nodes:

$ crsctl check cluster

CRS-4537: Cluster Ready Services is online

CRS-4529: Cluster Synchronization Services is online

CRS-4533: Event Manager is online

21. How to check the cluster (one node) status?

$ crsctl check crs

CRS-4638: Oracle High Availability Services is online

CRS-4537: Cluster Ready Services is online

CRS-4529: Cluster Synchronization Services is online

CRS-4533: Event Manager is online

22. How to find Voting Disk location?

•To determine the location of the voting disk:

# crsctl query css votedisk

## STATE File Universal Id File Name Disk group

– —– —————– ———- ———-

1. ONLINE 8c2e45d734c64f8abf9f136990f3daf8 (ASMDISK01) [DATA]

2. ONLINE 99bc153df3b84fb4bf071d916089fd4a (ASMDISK02) [DATA]

3. ONLINE 0b090b6b19154fc1bf5913bc70340921 (ASMDISK03) [DATA]

Located 3 voting disk(s).

23. How to find Location of OCR?

• cat /etc/oracle/ocr.loc

ocrconfig_loc=+DATA

local_only=FALSE

• #OCRCHECK (also about OCR integrity)

24. List some background process that used in ASM Instances?

Process	Description
RBAL	Opens all device files as part of discovery and coordinates the rebalance activity
ARBn	One or more slave processes that do the rebalance activity
GMON	Responsible for managing the disk-level activities such as drop or offline and advancing the ASM disk group compatibility
MARK	Marks ASM allocation units as stale when needed
Onnn	One or more ASM slave processes forming a pool of connections to the ASM instance for exchanging messages
PZ9n	One or more parallel slave processes used in fetching data on clustered ASM installation from GV$ views

25. What are types of ASM Mirroring?

Disk Group Type	Supported MirroringLevels	Default Mirroring Level
External redundancy	Unprotected (None)	Unprotected (None)
Normal redundancy	Two-wayThree-wayUnprotected (None)	Two-way
High redundancy	Three-way	Three-way

26. What is ASM Striping?

ASM can use variable size data extents to support larger files, reduce memory requirements, and improve performance.

Each data extent resides on an individual disk.

Data extents consist of one or more allocation units.

The data extent size is:

• Equal to AU for the first 20,000 extents (0–19999)
• Equal to 4 × AU for the next 20,000 extents (20000–39999)
• Equal to 16 × AU for extents above 40,000

ASM stripes files using extents with a coarse method for load balancing or a fine method to reduce latency.

• Coarse-grained striping is always equal to the effective AU size.
• Fine-grained striping is always equal to 128 KB.

27. How many ASM Diskgroups can be created under one ASM Instance?

ASM imposes the following limits:

• 63 disk groups in a storage system
• 10,000 ASM disks in a storage system
• Two-terabyte maximum storage for each ASM disk (non-Exadata)
• Four-petabyte maximum storage for each ASM disk (Exadata)
• 40-exabyte maximum storage for each storage system
• 1 million files for each disk group
• ASM file size limits (database limit is 128 TB):

1. External redundancy maximum file size is 140 PB.
2. Normal redundancy maximum file size is 42 PB.
3. High redundancy maximum file size is 15 PB.

28. How to find the cluster network settings?

To determine the list of interfaces available to the cluster:

$ oifcfg iflist –p -n

To determine the public and private interfaces that have been configured:

$ oifcfg getif

eth0 192.0.2.0 global public

eth1 192.168.1.0 global cluster_interconnect

To determine the Virtual IP (VIP) host name, VIP address, VIP subnet mask, and VIP interface name:

$ srvctl config nodeapps -a

VIP exists.:host01

VIP exists.: /192.0.2.247/192.0.2.247/255.255.255.0/eth0

…

29. How to change Cluster interconnect in RAC?

On a single node in the cluster, add the new global interface specification:

$ oifcfg setif -global eth2/192.0.2.0:cluster_interconnect

Verify the changes with oifcfg getif and then stop Clusterware on all nodes by running the following command as root on each node:

# oifcfg getif

# crsctl stop crs

Assign the network address to the new network adapters on all nodes using ifconfig:

#ifconfig eth2 192.0.2.15 netmask 255.255.255.0 broadcast 192.0.2.255

Remove the former adapter/subnet specification and restart Clusterware:

$ oifcfgdelif -global eth1/192.168.1.0

# crsctl start crs

30. Managing or Modifying SCAN in Oracle RAC?

To add a SCAN VIP resource:

$ srvctl add scan -n cluster01-scan

To remove Clusterware resources from SCAN VIPs:

$ srvctl remove scan [-f]

To add a SCAN listener resource:

$ srvctl add scan_listener

$ srvctl add scan_listener -p 1521

To remove Clusterware resources from all SCAN listeners:

$ srvctl remove scan_listener [-f]

31. How to check the node connectivity in Oracle Grid Infrastructure?

$ cluvfy comp nodecon -n all –verbose

32. Can I stop all nodes in one command? Meaning that stopping whole cluster ?

In 10g its not possible, where in 11g it is possible

[root@pic1]# crsctl start cluster -all
[root@pic2]# crsctl stop cluster –all

33. What is OLR? Which of the following statements regarding the Oracle Local Registry (OLR) is true?

1.Each cluster node has a local registry for node-specific resources.

2.The OLR should be manually created after installing Grid Infrastructure on each node in the cluster.

3.One of its functions is to facilitate Clusterware startup in situations where the ASM stores the OCR and voting disks.

4.You can check the status of the OLR using ocrcheck.

34. What is runfixup.sh script in Oracle Clusterware 11g release 2 installation

With Oracle Clusterware 11g release 2, Oracle Universal Installer (OUI) detects when the minimum requirements for an installation are not met, and creates shell scripts, called fixup scripts, to finish incomplete system configuration steps. If OUI detects an incomplete task, then it generates fixup scripts (runfixup.sh). You can run the fixup script after you click the Fix and Check Again Button.

The Fixup script does the following:

If necessary sets kernel parameters to values required for successful installation, including:

• Shared memory parameters.
• Open file descriptor and UDP send/receive parameters.

Sets permissions on the Oracle Inventory (central inventory) directory. Reconfigures primary and secondary group memberships for the installation owner, if necessary, for the Oracle Inventory directory and the operating system privileges groups.

• Sets shell limits if necessary to required values.

35. How to stop whole cluster with single command

crsctl stop cluster (possible only from 11gr2), please note crsctl commands becomes global now, if you do not specify node specifically the command executed globally for example

crsctl stop crs (stops in all crs resource in all nodes)

crsctl stop crs –n <ndeoname) (stops only in specified node)

36. CRS is not starting automatically after a node reboot, what you do to make it happen?

crsctl enable crs (as root)

to disable

crsctl disable crs (as root)

37. Describe high level Steps to convert single instance to RAC?

Read here

38. What is the difference between TAF and FAN & FCF? at what conditions you use them?

1) TAF with tnsnames
a feature of Oracle Net Services for OCI8 clients. TAF is transparent application failover which will move a session to a backup connection if the session fails. With Oracle 10g Release 2, you can define the TAF policy on the service using dbms_service package. It will only work with OCI clients. It will only move the session and if the parameter is set, it will failover the select statement. For insert, update or delete transactions, the application must be TAF aware and roll back the transaction. YES, you should enable FCF on your OCI client when you use TAF, it will make the failover faster.
Note: TAF will not work with JDBC thin.
2) FAN with tnsnames with aq notifications true

FAN is a feature of Oracle RAC which stands for Fast Application Notification. This allows the database to notify the client of any change (Node up/down, instance up/down, database up/down). For integrated clients, inflight transactions are interrupted and an error message is returned. Inactive connections are terminated. 
FCF is the client feature for Oracle Clients that have integrated with FAN to provide fast failover for connections. Oracle JDBC Implicit Connection Cache, Oracle Data Provider for .NET (ODP.NET) and Oracle Call Interface are all integrated clients which provide the Fast Connection Failover feature.
3) FCF, along with FAN when using connection pools
FCF is a feature of Oracle clients that are integrated to receive FAN events and abort inflight transactions, clean up connections when a down event is received as well as create new connections when a up event is received. Tomcat or JBOSS can take advantage of FCF if the Oracle connection pool is used underneath. This can be either UCP (Universal Connection Pool for JAVA) or ICC (JDBC Implicit Connection Cache). UCP is recommended as ICC will be deprecated in a future release.

4) ONS, with clusterware either FAN/FCF

ONS is part of the clusterware and is used to propagate messages both between nodes and to application-tiers
ONS is the foundation for FAN upon which is built FCF.
RAC uses FAN to publish configuration changes and LBA events. Applications can react as those published events in two way :
- by using ONS api (you need to program it)
- by using FCF (automatic by using JDBC implicit connection cache on the application server)
you can also respond to FAN event by using server-side callout but this on the server side (as their name suggests it)

Relationship between FAN/FCF/ONS

ONS –> FAN –> FCF
ONS -> send/receive messages on local and remote nodes.
FAN -> uses ONS to notify other processes about changes in configuration of service level
FCF -> uses FAN information working with conection pools JAVA and others.

39. Can you add voting disk online? Do you need voting disk backup?

Yes,  as per documentation, if you have multiple voting disk you can add online, but if you have only one voting disk , by that cluster will be down as its lost you just need to start crs in exclusive mode and add the votedisk using

crsctl add votedisk <path>

40. You have lost OCR disk, what is your next step?

The cluster stack will be down due to the fact that cssd is unable to maintain the integrity, this is true in 10g, From 11gR2 onwards, the crsd stack will be down, the hasd still up and running. You can add the ocr back by restoring the automatic backup or import the manual backup,

 Some practical questions added here

1. Viewing Contents in OCR/Voting disks

         There are three possible ways to view the OCR contents.

         a.       OCRDUMP (or)

         b.       crs_stat -p  (or)

         c.       By using strings.

         Voting disk contents are not persistent and are not required to view the contents, because the voting disk contents will be overwritten. if still need to view, strings are used.

2. Server pools – Read in my blog

3. Verifying Cluster Interconnect

                  

                 Cluster interconnects can be verified by:

         i.       oifcfg getif

         ii.      From AWR Report.

         iii.     show parameter cluster_interconnect

         iv.      srvctl config network

4. Does scan IP required or we can disable it

    

          SCAN IP can be disabled if not required. However SCAN IP is mandatory during the RAC installation. Enabling/disabling SCAN IP is mostly used in oracle apps environment by the concurrent manager (kind of job scheduler in oracle apps).

         To disable the SCAN IP,

         i.       Do not use SCAN IP at the client end.

         ii.      Stop scan listener

               srvctl stop scan_listener

         iii.     Stop scan

               srvctl stop scan (this will stop the scan vip's)

         iv.      Disable scan and disable scan listener

              srvctl disable scan

5. Migrating to new Diskgroup scenarious

a.       Case 1: Migrating disk group from one storage to other with same name

        1. Consider the disk group is DATA,

        2. Create new disks in DATA pointing towards the new storage (EMC),

                  a) Partioning provisioning done by storage and they give you the device name or mapper like /dev/mapper/asakljdlas

        3. Add the new disk to diskgroup DATA

                  a) Alter diskgroup data add disk '/dev/mapper/asakljdlas' 

        3. drop the old disks from DATA with which rebalancing is done automatically. 

        If you want you can the rebalance by alter system set asm_power_limit =12 for full throttle.

            alter diskgroup data drop disk 'path to hitachi storage'

            Note: you can get the device name in v$asm_disk in path column.

        4. Request SAN team to detach the old Storage (HITACHI).

b.       Case 2: Migrating disk group from one to another with different diskgroup name.

        1) Create the Disk group with new name in the new storage.

        2) Create the spfile in new diskgroup and change the parameter scope = spfile for control files etc.

        3) Take a control file backup in format +newdiskgroup

        4) Shutdown the db, startup nomount the database

        5) restore the control file from backup (now the control will restore to new diskgroup)

        6) Take the RMAN backup as copy of all the databases with new format.

               RMAN> backup database as copy format '+newdiskgroup name' ;

        3) RMAN> Switch database to copy.

        4) Verify dba_data_files,dba_temp_files, v$log that all files are pointing to new diskgroup name.

c.       Case 3: Migrating disk group to new storage but no additional diskgroup given

         1) Take the RMAN backup as copy of all the databases with new format and place it in the disk.

         2) Prepare rename commands from v$log ,v$datafile etc (dynamic queries)

         3) Take a backup of pfile and modify the following referring to new diskgroup name

                  .control_files

                 .db_create_file_dest

                 .db_create_online_log_dest_1

                 .db_create_online_log_dest_2

                 .db_recovery_file_des

                          4) stop the database

                          5) Unmount the diskgroup

                               asmcmd umount ORA_DATA

                          6) use asmcmd renamedg (11gr2 only) command to rename to new diskgroup

                               renamedg phase=both dgname=ORA_DATA newdgname=NEW_DATA verbose=true 

                 7)  mount the diskgroup

                      asmcmd mount NEW_DATA

                 8) start the database in mount with new pfile taken backup in step 3

                 9) Run the rename file scripts generated at step2

                 9) Add the diskgroup to cluster the cluster (if using rac)

                 srvctl modify database -d orcl -p +NEW_FRA/orcl/spfileorcl.ora

                          srvctl modify database -d orcl -a "NEW_DATA"

                          srvctl config database -d orcl  

                          srvctl start database -d orcl

                                        10) Delete the old diskgroup from cluster

                        crsctl delete resource ora.ORA_DATA.dg

                11) Open the database.

7. Database rename in RAC, what could be the checklist for you?

         a.       Take the outputs of all the services that are running on the databases.

         b.       set cluster_database=FALSE

         c.       Drop all the services associated with the database.

         d.       Stop the database

         e.       Startup mount

         f.       Use nid to change the DB Name.

                   Generic question, If using ASM the usual location for the datafile would be +DATA/datafile/OLDDBNAME/system01.dbf'

                   Does NID changes this path too? to reflect the new db name?

                   Yes it will, by using proper directory structure it will create a links to original directory structure. +DATA/datafile/NEWDBNAME/system01.dbf'

                   this has to be tested,  We dont have test bed, but thanks to Anji who confirmed it will

         g.       Change the parameters according to the new database name

         h.       Change the password file.

         i.       Stop the database.

         j.       Mount the database

         k.       Open database with Reset logs

         l.       Create spfile from pfile.

         m.       Add database to the cluster.

         n.       Create the services that are dropped in prior to rename.

         o.       Bounce the database.

8.How to find the database in which particular service is attached to when you have a large number of databases running in the server, you cannot check one by one manually

Write a shell script to read the database name from oratab and iterate the loop taking inpt as DB name in srvctl to get the result.
#!/bin/ksh
ORACLE_HOME=
PATH=$ORACLE_HOME/bin:$PATH
LD_LIBRARY_PATH=${SAVE_LLP}:${ORACLE_HOME}/lib
export TNS_ADMIN ORACLE_HOME PATH LD_LIBRARY_PATH
for INSTANCE in `cat /etc/oratab|grep -v "^#"|cut -f1 -d: -s`
do
export ORACLE_SID=$INSTANCE
echo `srvctl status service -d $INSTANCE -s $1| grep -i "is running"`
done

9. Difference between OHAS and CRS
OHAS is complete cluster stack which includes some kernel level tasks like managing network,time synchronization, disks etc, where the CRS has the ability to manage the resources like database,listeners,applications, etc With both of this Oracle provides the high availability

Oracle RAC Interview Questions & Answers

1. Where are the Clusterware files stored on a RAC environment?
The Clusterware is installed on each node (on an Oracle Home) and on the shared disks (the voting disks and the CSR file)

2. Where are the database software files stored on a RAC environment?

The base software is installed on each node of the cluster and the
database storage on the shared disks.

3. What kind of storage we can use for the shared Clusterware files?
- OCFS (Release 1 or 2)
- raw devices
- third party cluster file system such as GPFS or Veritas

4. What kind of storage we can use for the RAC database storage?
- OCFS (Release 1 or 2)
- ASM
- raw devices
- third party cluster file system such as GPFS or Veritas

5. What is a CFS?
A cluster File System (CFS) is a file system that may be accessed (read and write) by all members in a cluster at the same time. This implies that all members of a cluster have the same view.

6. What is an OCFS2?
The OCFS2 is the Oracle (version 2) Cluster File System which can be used for the Oracle Real Application Cluster.

7. Which files can be placed on an Oracle Cluster File System?
- Oracle Software installation (Windows only)
- Oracle files (controlfiles, datafiles, redologs, files described by the bfile datatype)
- Shared configuration files (spfile)
- OCR and voting disk
- Files created by Oracle during runtime

Note: There are some platform specific limitations.

8. Do you know another Cluster Vendor?
HP Tru64 Unix, Veritas, Microsoft

9. How is possible to install a RAC if we don’t have a CFS?
This is possible by using a raw device.

10. What is a raw device?
A raw device is a disk drive that does not yet have a file system set up. Raw devices are used for Real Application Clusters since they enable the sharing of disks.

11. What is a raw partition?
A raw partition is a portion of a physical disk that is accessed at the lowest possible level. A raw partition is created when an extended partition is created and logical partitions are assigned to it without any formatting. Once formatting is complete, it is called cooked partition.

12. When to use CFS over raw?
A CFS offers:
- Simpler management
- Use of Oracle Managed Files with RAC
- Single Oracle Software installation
- Autoextend enabled on Oracle datafiles
- Uniform accessibility to archive logs in case of physical node failure
- With Oracle_Home on CFS, when you apply Oracle patches CFS guarantees that the updated Oracle_Home is visible to all nodes in the cluster.

Note: This option is very dependent on the availability of a CFS on your platform.

13. When to use raw over CFS?
- Always when CFS is not available or not supported by Oracle.
- The performance is very, very important: Raw devices offer best performance without any intermediate layer between Oracle and the disk.
Note: Autoextend fails on raw devices if the space is exhausted. However the space could be added online if needed.

14. What CRS is?
Oracle RAC 10g Release 1 introduced Oracle Cluster Ready Services (CRS), a platform-independent set of system services for cluster environments. In Release 2, Oracle has renamed this product to Oracle Clusterware.

15. What is VIP IP used for?
It returns a dead connection IMMIDIATELY, when its primary node fails. Without using VIP IP, the clients have to wait around 10 minutes to receive ORA-3113: “end of file on communications channel”. However, using Transparent Application Failover (TAF) could avoid ORA-3113.

16. Why we need to have configured SSH or RSH on the RAC nodes?
SSH (Secure Shell,10g+) or RSH (Remote Shell, 9i+) allows “oracle” UNIX account connecting to another RAC node and copy/ run commands as the local “oracle” UNIX account.

17. Is the SSH, RSH needed for normal RAC operations?
No. SSH or RSH are needed only for RAC, patch set installation and clustered database creation.

18. Do we have to have Oracle RDBMS on all nodes?
Each node of a cluster that is being used for a clustered database will typically have the RDBMS and RAC software loaded on it, but not actual data files (these need to be available via shared disk).

19. What are the restrictions on the SID with a RAC database? Is it limited to 5 characters?
The SID prefix in 10g Release 1 and prior versions was restricted to five characters by install/ config tools so that an ORACLE_SID of up to max of 5+3=8 characters can be supported in a RAC environment. The SID prefix is relaxed up to 8 characters in 10g Release 2, see bug 4024251 for more information.

20. Does Real Application Clusters support heterogeneous platforms?
The Real Application Clusters do not support heterogeneous platforms in the same cluster.

21. Are there any issues for the interconnect when sharing the same switch as the public network by using VLAN to separate the network?
RAC and Clusterware deployment best practices suggests that the interconnect (private connection) be deployed on a stand-alone, physically separate, dedicated switch. On big network the connections could be instable.

22. What is the Load Balancing Advisory?
To assist in the balancing of application workload across designated resources, Oracle Database 10g Release 2 provides the Load Balancing Advisory. This Advisory monitors the current workload activity across the cluster and for each instance where a service is active; it provides a percentage value of how much of the total workload should be sent to this instance as well as service quality flag.

23. How many nodes are supported in a RAC Database?
With 10g Release 2, we support 100 nodes in a cluster using Oracle Clusterware, and 100 instances in a RAC database. Currently DBCA has a bug where it will not go beyond 63 instances. There is also a documentation bug for the max-instances parameter. With 10g Release 1 the Maximum is 63.

24. What is the Cluster Verification Utiltiy (cluvfy)?
The Cluster Verification Utility (CVU) is a validation tool that you can use to check all the important components that need to be verified at different stages of deployment in a RAC environment.

25. What versions of the database can I use the cluster verification utility (cluvfy) with?
The cluster verification utility is release with Oracle Database 10g Release 2 but can also be used with Oracle Database 10g Release 1.

26. If I am using Vendor Clusterware such as Veritas, IBM, Sun or HP, do I still need Oracle Clusterware to run Oracle RAC 10g?
Yes. When certified, you can use Vendor Clusterware however you must still install and use Oracle Clusterware for RAC. Best Practice is to leave Oracle Clusterware to manage RAC. For details see Metalink Note 332257.1 and for Veritas SFRAC see 397460.1.

27. Is RAC on VMWare supported?
Yes.

28. What is hangcheck timer used for ? 
The hangcheck timer checks regularly the health of the system. If the system hangs or stop the node will be restarted automatically.
There are 2 key parameters for this module:
-> hangcheck-tick: this parameter defines the period of time between checks of system health. The default value is 60 seconds; Oracle recommends setting it to 30seconds.
-> hangcheck-margin: this defines the maximum hang delay that should be tolerated before hangcheck-timer resets the RAC node.

29. Is the hangcheck timer still needed with Oracle RAC 10g?
Yes.

30. What files can I put on Linux OCFS2?
For optimal performance, you should only put the following files on Linux OCFS2:
- Datafiles
- Control Files
- Redo Logs
- Archive Logs
- Shared Configuration File (OCR)
- Voting File
- SPFILE

31. Is it possible to use ASM for the OCR and voting disk?
No, the OCR and voting disk must be on raw or CFS (cluster file system).

32. Can I change the name of my cluster after I have created it when I am using Oracle Clusterware?
No, you must properly uninstall Oracle Clusterware and then re-install.

33. What the O2CB is?
The O2CB is the OCFS2 cluster stack. OCFS2 includes some services. These services must be started before using OCFS2 (mount/ format the file systems).

34. What the OCR file is used for? 
OCR is a file that manages the cluster and RAC configuration.

35. What the Voting Disk file is used for? 
The voting disk is nothing but a file that contains and manages information of all the node memberships.

36. What is the recommended method to make backups of a RAC environment?                          RMAN to make backups of the database, dd to backup your voting disk and hard copies of the OCR file.

37.  What command would you use to check the availability of the RAC system?
crs_stat -t -v (-t -v are optional)
38. What is the minimum number of instances you need to have in order to create a RAC?
You can create a RAC with just one server.
39.  Name two specific RAC background processes
RAC processes are: LMON, LMDx, LMSn, LKCx and DIAG.

40.  Can you have many database versions in the same RAC?
Yes, but Clusterware version must be greater than the greater database version.

41.  What was RAC previous name before it was called RAC?OPS: Oracle Parallel Server

42.  What RAC component is used for communication between instances?Private Interconnect.

43.  What is the difference between normal views and RAC views?A RAC view has the prefix ‘G’. For example, GV$SESSION instead of V$SESSION

44.  Which command will we use to manage (stop, start) RAC services in command-line mode?
srvctl

45.  How many alert logs exist in a RAC environment?
A- One for each instance.

46. What are Oracle Clusterware Components

Voting Disk — Oracle RAC uses the voting disk to manage cluster membership by way of a health check and arbitrates cluster ownership among the instances in case of network failures. The voting disk must reside on shared disk.

Oracle Cluster Registry (OCR) — Maintains cluster configuration information as well as configuration information about any cluster database within the cluster. The OCR must reside on shared disk that is accessible by all of the nodes in your cluster

47. How do you backup voting disk

#dd if=voting_disk_name of=backup_file_name

48. How do I identify the voting disk location

#crsctl query css votedisk

49. How do I identify the OCR file location

check /var/opt/oracle/ocr.loc or /etc/ocr.loc ( depends upon platform)
or
#ocrcheck

50. What is SCAN?

Single Client Access Name (SCAN) is s a new Oracle Real Application Clusters (RAC) 11g Release 2 feature that provides a single name for clients to access an Oracle Database running in a cluster. The benefit is clients using SCAN do not need to change if you add or remove nodes in the cluster.

                                              ALL THE BEST


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