How SCAN works?

1. PMON process of each instance registers the database services with the default listener on the local node and with each SCAN listener, which is specified by the REMOTE_LISTENER database parameter.
2. Oracle client connects using SCAN name: myscan:1521/sales.example.com
3. Client queries DNS to resolve scan_name.
4. SCAN listener selects least loaded node (node in this example)
5. The client connects to the local listener on node2. The local listener starts a dedicated server process for the connection to the database.
6. The client connects directly to the dedicated server process on node2 and accesses the sales2 database instance.                                                         Ref: http://www.freeoraclehelp.com/2011/12/scan-setup-for-oracle-11g-release211gr2.html

RAC Questions

What command would you use to check the availability of the RAC system?

crs_stat -t -v (-t -v are optional)

How do we verify that RAC instances are running?

SQL>select * from V$ACTIVE_INSTANCES;

The query gives the instance number under INST_NUMBER column,host_:instancename under INST_NAME column.

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

How do you backup the OCR ?

There is an automatic backup mechanism for OCR. The default location is : $ORA_CRS_HOME\cdata\"clustername"\

To display backups :

#ocrconfig -showbackup

To restore a backup :

#ocrconfig -restore

With Oracle RAC 10g Release 2 or later, you can also use the export command:

#ocrconfig -export -s online, and use -import option to restore the contents back.

With Oracle RAC 11g Release 1, you can do a manaual backup of the OCR with the command:

# ocrconfig -manualbackup

How do you backup voting disk ?

#dd if=voting_disk_name of=backup_file_name

How do I identify the voting disk location ?

#crsctl query css votedisk

How do I identify the OCR file location ?

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

or

#ocrcheck

Is ssh required for normal Oracle RAC operation ?

"ssh" are not required for normal Oracle RAC operation. However "ssh" should be enabled for Oracle RAC and patchset installation.

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. Click here for more details from Oracle

What is the purpose of Private Interconnect ?

Clusterware uses the private interconnect for cluster synchronization (network heartbeat) and daemon communication between the the clustered nodes. This communication is based on the TCP protocol.RAC uses the interconnect for cache fusion (UDP) and inter-process communication (TCP). Cache Fusion is the remote memory mapping of Oracle buffers, shared between the caches of participating nodes in the cluster.

 Why do we have a Virtual IP (VIP) in Oracle RAC?   

Without using VIPs or FAN, clients connected to a node that died will often wait for a TCP timeout period (which can be up to 10 min) before getting an error. As a result, you don't really have a good HA solution without using VIPs. When a node fails, the VIP associated with it is automatically failed over to some other node and new node re-arps the world indicating a new MAC address for the IP. Subsequent packets sent to the VIP go to the new node, which will send error RST packets back to the clients. This results in the clients getting errors immediately.

How many nodes are supported in a RAC Database?

10g Release 2, support 100 nodes in a cluster using Oracle Clusterware, and 100 instances in a RAC database.

What is voting disk?

Voting Disk is a file that sits in the shared storage area and must be accessible by all nodes in the cluster. All nodes in the cluster registers their heart-beat information in the voting disk, so as to confirm that they are all operational. If heart-beat information of any node in the voting disk is not available that node will be evicted from the cluster. The CSS (Cluster Synchronization Service) daemon in the clusterware maintains the heart beat of all nodes to the voting disk. When any node is not able to send heartbeat to voting disk, then it will reboot itself, thus help avoiding the split-brain syndrome.

For high availability, Oracle recommends that you have a minimum of three or odd number (3 or greater) of votingdisks.

Voting Disk – is file that resides on shared storage and Manages cluster members. Voting disk reassigns cluster ownership between the nodes in case of failure.

The Voting Disk Files are used by Oracle Clusterware to determine which nodes are currently members of the cluster. The voting disk files are also used in concert with other Cluster components such as CRS to maintain the clusters integrity.

Oracle Database 11g Release 2 provides the ability to store the voting disks in ASM along with the OCR. Oracle Clusterware can access the OCR and the voting disks present in ASM even if the ASM instance is down. As a result CSS can continue to maintain the Oracle cluster even if the ASM instance has failed.

How many voting disks are you maintaining ?

By default Oracle will create 3 voting disk files in ASM.

Oracle expects that you will configure at least 3 voting disks for redundancy purposes. You should always configure an odd number of voting disks >= 3. This is because loss of more than half your voting disks will cause the entire cluster to fail.

You should plan on allocating 280MB for each voting disk file. For example, if you are using ASM and external redundancy then you will need to allocate 280MB of disk for the voting disk. If you are using ASM and normal redundancy you will need 560MB.

Why we need to keep odd number of voting disks ?

Oracle expects that you will configure at least 3 voting disks for redundancy purposes. You should always configure an odd number of voting disks >= 3. This is because loss of more than half your voting disks will cause the entire cluster to fail.

What is SCAN? (11gR2 feature)

Single Client Access Name (SCAN) eliminates the need to change TNSNAMES entry when nodes are added to or removed from the Cluster. RAC instances register to SCAN listeners as remote listeners. Oracle recommends assigning 3 addresses to SCAN, which will create 3 SCAN listeners, though the cluster has got dozens of nodes.. SCAN is a domain name registered to at least one and up to three IP addresses, either in DNS (Domain Name Service) or GNS (Grid Naming Service). The SCAN must resolve to at least one address on the public network. For high availability and scalability, Oracle recommends configuring the SCAN to resolve to three addresses.

What are SCAN components in a cluster?

1.SCAN Name

2.SCAN IPs (3)

3.SCAN Listeners (3)

What are the requirements for Oracle Clusterware?

1. External Shared Disk to store Oracle Cluster ware file (Voting Disk and Oracle Cluster Registry – OCR)

2. Two netwrok cards on each cluster ware node (and three set of IP address) -

Network Card 1 (with IP address set 1) for public network

Network Card 2 (with IP address set 2) for private network (for inter node communication between rac nodes used by clusterware and rac database)

IP address set 3 for Virtual IP (VIP) (used as Virtual IP address for client connection and for connection failover)

3. Storage Option for OCR and Voting Disk – RAW, OCFS2 (Oracle Cluster File System), NFS, …..

How to find location of OCR file when CRS is down?

If you need to find the location of OCR (Oracle Cluster Registry) but your CRS is down.

When the CRS is down:

Look into “ocr.loc” file, location of this file changes depending on the OS:

On Linux: /etc/oracle/ocr.loc

On Solaris: /var/opt/oracle/ocr.loc

When CRS is UP:

Set ASM environment or CRS environment then run the below command:

ocrcheck

In 2 node RAC, how many NIC’s are r using ?

2 network cards on each clusterware node

Network Card 1 (with IP address set 1) for public network

Network Card 2 (with IP address set 2) for private network (for inter node communication between rac nodes used by clusterware and rac database)

In 2 node RAC, how many IP’s are r using ?

6 – 3 set of IP address

## eth1-Public: 2

## eth0-Private: 2

## VIP: 2

How to find IP’s information in RAC ?

Edit the /etc/hosts file as shown below:

# Do not remove the following line, or various programs

# that requires network functionality will fail.

127.0.0.1 localhost.localdomain localhost

## Public Node names

192.168.10.11 node1-pub.hingu.net node1-pub

192.168.10.22 node2-pub.hingu.net node2-pub

## Private Network (Interconnect)

192.168.0.11 node1-prv node1-prv

192.168.0.22 node2-prv node2-prv

## Private Network (Network Area storage)

192.168.1.11 node1-nas node1-nas

192.168.1.22 node2-nas node2-nas

192.168.1.33 nas-server nas-server

## Virtual IPs

192.168.10.111 node1-vip.hingu.net node1-vip

192.168.10.222 node2-vip.hingu.net node2-vip

What is difference between RAC ip addresses ?

Public IP adress is the normal IP address typically used by DBA and SA to manage storage, system and database. Public IP addresses are reserved for the Internet.

Private IP address is used only for internal clustering processing (Cache Fusion) (aka as interconnect). Private IP addresses are reserved for private networks.

VIP is used by database applications to enable fail over when one cluster node fails. The purpose for having VIP is so client connection can be failover to surviving nodes in case there is failure

Can application developer access the private ip ?

No. private IP address is used only for internal clustering processing (Cache Fusion) (aka as interconnect)

How many OCR and voting disks should one have?

For redundancy, one should have at least two OCR disks and three voting disks (raw disk partitions).

These disk partitions should be spread across different physical disks.

  

Why we required to maintain odd number of voting disks?

Odd number of disk are to avoid split brain,When Nodes in cluster can't talk to each other they run to lock the Voting disk and whoever lock the more disk will survive,if disk number are even there are chances that node might lock 50% of disk (2 out of 4) then how to decide which node to evict.whereas when number is odd, one will be higher than other and each for cluster to evict the node with less number.

How you check the health of Your RAC Database?

 'crsctl' command from root or oracle user can be used to check the clusterware health But for starting or stopping we have to use root user or any privilege user.

[oracle@TEST_NODE1 ~]$ crsctl check crs

CSS appears healthy

CRS appears healthy

EVM appears healthy

How you will backup your RAC Database?

Backup strategy of RAC Database:An RAC Database consists of

1)OCR

2)Voting disk 

3)Database files, controlfiles, redolog files & Archive log files

Give the usage of srvctl ? 

srvctl start instance -d db_name -i "inst_name_list" [-o start_options]

srvctl stop instance -d name -i "inst_name_list" [-o stop_options]

srvctl stop instance -d orcl -i "orcl3,orcl4" -o immediate

srvctl start database -d name [-o start_options]

srvctl stop database -d name [-o stop_options]

srvctl start database -d orcl -o mount

What are the major RAC wait events?

In a RAC environment the buffer cache is global across all instances in the cluster and hence the processing differs.

The most common wait events related to this are gc cr request

and gc buffer busy

 GC CR request :the time it takes to retrieve the data from the remote cache

Reason: RAC Traffic Using Slow Connection or Inefficient queries (poorly tuned queries will increase the amount of data blocks requested by an Oracle session.

The more blocks requested typically means the more often a block will need to be read from a remote instance via the interconnect.)

GC BUFFER BUSY: It is the time the remote instance locally spends accessing the requested data block.

How do we verify an existing current backup of OCR?

We can verify the current backup of OCR using the following command : ocrconfig -showbackup

How to move OCR and Voting disk to new storage device? ( verify in non-prod first)

Moving OCR

==========

You must be logged in as the root user, because root owns the OCR files.

 Also an ocrmirror must be in place before trying to replace the OCR device.

Make sure there is a recent backup of the OCR file before making any changes:

ocrconfig –showbackup

If there is not a recent backup copy of the OCR file, an export can be taken for the current OCR file. Use the following command to generate an export of the online OCR file:

In 10.2

# ocrconfig –export -s online

In 11g

# ocrconfig -manualbackup

The new OCR disk must be owned by root, must be in the oinstall group, and must have permissions set to 640. Provide at least 100 MB disk space for the OCR.

On one node as root run:

# ocrconfig -replace ocr

# ocrconfig -replace ocrmirror

Now run ocrcheck to verify if the OCR is pointing to the new file

Moving Voting Disk

==================

Note: crsctl votedisk commands must be run as root

Shutdown the Oracle Clusterware (crsctl stop crs as root) on all nodes before making any modification to the voting disk. Determine the current voting disk location using:

crsctl query css votedisk

Take a backup of all voting disk:

dd if=voting_disk_name of=backup_file_name

To move a Voting Disk, provide the full path including file name:

crsctl delete css votedisk –force

crsctl add css votedisk –force

After modifying the voting disk, start the Oracle Clusterware stack on all nodes

# crsctl start crs

Verify the voting disk location using

crsctl query css votedisk

What is split brain ?

In RAC environment, server nodes communicate with each other using High speed private interconnects network. A split brain situation happens when all the links of the private interconnect fail to respond to each other but instances are still up and running. So each instance thinks that the other nodes/instances are dead and that it should take over the ownership.

In split brain situation, instances independtly access the data and modify the same blocks and the database will end up with changed database overwritten which could lead to data corruption. To avoid this, various algorithm are implemented to handle split brain scenario.

In RAC, the IMR (Instance Membership Recovery) service is one of the one of the efficient algorithm used to detect & resolve the split-brain syndrome. When one instance fails to communicate with other instances or when one instance becomes inactive due to any reason and is unable to issue the control file heartbeat, the split brain is detected and the detecting instance will evict the failed instance from the database.This process is called node eviction.

What is FAN?

Applications can use Fast Application Notification (FAN) to enable rapid failure detection, balancing of connection pools after failures, and re-balancing of connection pools when failed components are repaired.The FAN process uses system events that Oracle publishes when cluster servers become unreachable or if network interfaces fail.

What is FCF?

Fast Connection Failover provides high availability to FAN integrated clients, such as clients that use JDBC, OCI, or ODP.NET. If you configure the client to use fast connection failover, then the client automatically subscribes to FAN events and can react to database UP and DOWN events.

In response, Oracle gives the client a connection to an active instance that provides the  requested database service.

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

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

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

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

How do you troubleshoot node reboot ? 

Please check metalink ...

Note 265769.1 Troubleshooting CRS Reboots

Note.559365.1 Using Diagwait as a diagnostic to get more information for diagnosing Oracle Clusterware Node evictions. 

====================================================================================

What is split brain?

When database nodes in a cluster are unable to communicate with each other, they may continue to process and modify the data blocks independently. If the 

same block is modified by more than one instance, synchronization/locking of the data blocks does not take place and blocks may be overwritten by others in the cluster. This state is called split brain.

What is cache fusion?

In a RAC environment, it is the combining of data blocks, which are shipped across the interconnect from remote database caches (SGA) to the local node, in order to fulfill the requirements for a transaction (DML, Query of Data Dictionary).

What is the difference between Crash recovery and Instance recovery?

When an instance crashes in a single node database on startup a crash recovery takes place. In a RAC enviornment the same recovery for an instance is performed by the surviving nodes called Instance recovery.

What is the interconnect used for?

It is a private network which is used to ship data blocks from one instance to another for cache fusion. The physical data blocks as well as data dictionary blocks are shared across this interconnect.

How do you determine what protocol is being used for Interconnect traffic?

One of the ways is to look at the database alert log for the time period when the database was started up.

What methods are available to keep the time synchronized on all nodes in the cluster?

Either the Network Time Protocol(NTP) can be configured or in 11gr2, Cluster Time Synchronization Service (CTSS) can be used.

What files components in RAC must reside on shared storage?

Spfiles, ControlFiles, Datafiles and Redolog files should be created on shared storage.

Where does the Clusterware write when there is a network or Storage missed heartbeat?

The network ping failure is written in $CRS_HOME/log

How do you find out what OCR backups are available?

The ocrconfig -showbackup can be run to find out the automatic and manually run backups.

If your OCR is corrupted what options do have to resolve this?

You can use either the logical or the physical OCR backup copy to restore the Repository.

How do you find out what object has its blocks being shipped across the instance the most?

You can use the dba_hist_seg_stats.

What is a VIP in RAC use for?

The VIP is an alternate Virtual IP address assigned to each node in a cluster. During a node failure the VIP of the failed node moves to the surviving node and relays to the application that the node has gone down. Without VIP, the application will wait for TCP timeout and then find out that the session is no longer live due to the failure.

How do we know which database instances are part of a RAC cluster?

You can query the V$ACTIVE_INSTANCES view to determine the member instances of the RAC cluster.

What is OCLUMON used for in a cluster environment?

The Cluster Health Monitor (CHM) stores operating system metrics in the CHM repository for all nodes in a RAC cluster. It stores information on CPU, memory, process, network and other OS data, This information can later be retrieved and used to troubleshoot and identify any cluster related issues. It is a default component of the 11gr2 grid install. The data is stored in the master repository and replicated to a standby repository on a different node.

What would be the possible performance impact in a cluster if a less powerful node (e.g. slower CPU’s) is added to the cluster?

All processing will show down to the CPU speed of the slowest server.

What is the purpose of OLR?

Oracle Local repository contains information that allows the cluster processes to be started up with the OCR being in the ASM storage ssytem. Since the ASM file system is unavailable until the Grid processes are started up a local copy of the contents of the OCR is required which is stored in the OLR.

What is the default memory allocation for ASM?

In 10g the default SGA size is 1G in 11g it is set to 256M and in 12c ASM it is set back to 1G.

How do you backup ASM Metadata?

You can use md_backup to restore the ASM diskgroup configuration in-case of ASM diskgroup storage loss.

What files can be stored in the ASM diskgroup?

In 11g the following files can be stored in ASM diskgroups.

• Datafiles
• Redo logfiles
• Spfiles

In 12c the files below can also new be stored in the ASM Diskgroup

• Password file

What it the ASM POWER_LIMIT?

This is the parameter which controls the number of Allocation units the ASM instance will try to rebalance at any given time. In ASM versions less than 11.2.0.3 the default value is 11 however it has been changed to unlimited in later versions.

What is a rolling upgrade?

A patch is considered a rolling if it is can be applied to the cluster binaries without having to shutting down the database in a RAC environment. All nodes in the cluster are patched in a rolling manner, one by one, with only the node which is being patched unavailable while all other instance open.

What are some of the RAC specific parameters?

Some of the RAC parameters are:

• CLUSTER_DATABASE
• CLUSTER_DATABASE_INSTANCE
• INSTANCE_TYPE (RDBMS or ASM)
• ACTIVE_INSTANCE_COUNT
• UNDO_MANAGEMENT

What is the future of the Oracle Grid?

The Grid software is becoming more and more capable of not just supporting HA for Oracle Databases but also other applications including Oracle’s applications. With 12c there are more features and functionality built-in and it is easier to deploy these pre-built solutions, available for common Oracle applications.

What components of the Grid should I back up?

The backups should include OLR, OCR and ASM Metadata.

Is there an easy way to verify the inventory for all remote nodes

You can run the opatch lsinventory -all_nodes command from a single node to look at the inventory details for all nodes in the cluster.