Categories: RAC

Oracle RAC important questions

If you need to know more basic and some higher level things of RAC this article “Oracle RAC important questions” will help you out:

  • What is RAC?

RAC stands for Real Application cluster. It is a clustering solution from Oracle Corporation that ensures high availability of databases by providing instance failover, media failover features.

  • What is RAC and how is it different from non RAC databases?

RAC stands for Real Application Cluster, you have n number of instances running in their own separate nodes and based on the shared storage. Cluster is the key component and is a collection of servers operations as one unit. RAC is the best solution for high performance and High availability. Non RAC databases has single point of failure in case of hardware failure or server crash.

  • 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

  • Mention the Oracle RAC software components :

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

  • What is GRD?

GRD stands for Global Resource Directory. The GES and GCS maintains records of the statuses of each datafile and each cached block using global resource directory. This process is referred to as cache fusion and helps in data integrity.

  • What are the different network components are in 10g RAC?

Public, Private, and VIP components
Private interfaces is for Intra node communication. VIP is all about availability of application. When a node fails then the VIP component fail over to some other node, this is the reason that all applications should based on VIP components means TNS entries should have VIP entry in the host list

  • Give Details on ACMS:

ACMS stands for Atomic Controlfile Memory Service. In an Oracle RAC environment ACMS is an agent that ensures a distributed SGA memory update(ie)SGA updates are globally committed on success or globally aborted in event of a failure.

  • What is Cache Fusion?

Cache fusion is the mechanism to transfer the data block from memory to memory of one node to the other. If two nodes require the same block for query or update, the block must be transferred from the cache of one node to the other. RAC system must equipped with low-latency and high speed inter-connect to make it happen.

  • Give Details on Cache Fusion:

Oracle RAC is composed of two or more instances. When a block of data is read from datafile by an instance within the cluster and another instance is in need of the same block, it is easy to get the block image from the instance which has the block in its SGA rather than reading from the disk. To enable inter instance communication Oracle RAC makes use of interconnects. The Global Enqueue Service(GES) monitors and Instance enqueue process manages the cache fusion.
Cache Fusion is essentially a memory-to-memory transfer of data between the nodes in the RAC environment. Before Cache Fusion, a node was required to write some of the data to disk before it could be transferred to the next node in the cluster. Cache Fusion does a straight memory-to-memory transfer. In addition, each node’s SGA has a map of what data is contained in the other node’s data caches.
The performance improvement is phenomenal. Oracle leverages the vendor’s high speed interconnects between the nodes to achieve the cache-to-cache data transfers. Before Cache Fusion, when you added a node to the cluster to increase performance of the application, it didn’t always provide you with the performance improvement that you hoped for. With Cache Fusion, you can easily cost justify the addition of another node into a RAC cluster to increase the performance of the application running on it. Oracle sales pitches describe it as ‘near linear horizontal scalability’.

  • 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.

Give details on GTX0-j :

The process provides transparent support for XA global transactions in a RAC environment. The database auto tunes the number of these processes based on the workload of XA global transactions.

  • Give details on LMON:

This process monitors global enques and resources across the cluster and performs global enqueue recovery operations. This is called as Global Enqueue Service Monitor.
Give details on LMD:

This process is called as global enqueue service daemon. This process manages incoming remote resource requests within each instance.

  • Give details on LMS:

This process is called as Global Cache service process. This process maintains statuses of datafiles and each cached block by recording information in a Global Resource Directory (GRD).This process also controls the flow of messages to remote instances and manages global data block access and transmits block images between the buffer caches of different instances. This processing is a part of cache fusion feature.

  • Give details on LCK0:

This process is called as Instance enqueue process. This process manages non-cache fusion resource requests such as library and row cache requests.

  • Give details on RMSn:

This process is called as Oracle RAC management process. These processes perform manageability tasks for Oracle RAC. Tasks include creation of resources related Oracle RAC when new instances are added to the cluster.

  • Give details on RSMN:

This process is called as Remote Slave Monitor. This process manages background slave process creation and communication on remote instances. This is a background slave process. This process performs tasks on behalf of a coordinating process running in another instance.

  • What components in RAC must reside in shared storage?

All datafiles, controlfiles, SPFIles, redo log files must reside on cluster-aware shared storage.

  • What is the significance of using cluster-aware shared storage in an Oracle RAC environment?

All instances of an Oracle RAC can access all the datafiles, control files, SPFILE’s, redolog files when these files are hosted out of cluster-aware shared storage which are group of shared disks.

  • Give few examples for solutions that support cluster storage:

ASM (automatic storage management), raw disk devices, network file system (NFS), OCFS2 and OCFS (Oracle Cluster Fie systems).

  • What is an interconnect network?

An interconnect network is a private network that connects all of the servers in a cluster. The interconnect network uses a switch/multiple switches that only the nodes in the cluster can access.

  • How can we configure the cluster interconnect?

Configure User Datagram Protocol(UDP) on Gigabit Ethernet for cluster interconnect. On Unix and Linux systems we use UDP and RDS (Reliable data socket) protocols to be used by Oracle Clusterware. Windows clusters use the TCP protocol.

  • Can we use crossover cables with Oracle Clusterware interconnects?

No, crossover cables are not supported with Oracle Clusterware interconnects.

  • What is the use of cluster interconnect?

Cluster interconnect is used by the Cache fusion for inter instance communication.

  • How do users connect to database in an Oracle RAC environment?

Users can access a RAC database using a client/server configuration or through one or more middle tiers ,with or without connection pooling. Users can use oracle services feature to connect to database.

  • What is the use of a service in Oracle RAC environment?

Applications should use the services feature to connect to the Oracle database. Services enable us to define rules and characteristics to control how users and applications connect to database instances.

  • What are the characteristics controlled by Oracle services feature?

The characteristics include a unique name, workload balancing and failover options, and high availability characteristics.

  • What enables the load balancing of applications in RAC?

Oracle Net Services enable the load balancing of application connections across all of the instances in an Oracle RAC database.

  • What is a Virtual IP address or VIP?

A virtual IP address or VIP is an alternate IP address that the client connections use instead of the standard public IP address. To configure VIP address, we need to reserve a spare IP address for each node, and the IP addresses must use the same subnet as the public network.

  • What is the use of VIP?

If a node fails, then the node’s VIP address fails over to another node on which the VIP address can accept TCP connections but it cannot accept Oracle connections.

  • Give situations under which VIP address failover happens:

VIP addresses failover happens when the node on which the VIP address runs fails, all interfaces for the VIP address fails, all interfaces for the VIP address are disconnected from the network.

  • What is the significance of VIP address failover?

When a VIP address failover happens, Clients that attempt to connect to the VIP address receive a rapid connection refused error .They don’t have to wait for TCP connection timeout messages.

  • What are the administrative tools used for Oracle RAC environments?

Oracle RAC cluster can be administered as a single image using OEM (Enterprise Manager), SQL*PLUS,Servercontrol(SRVCTL),clusterverificationutility(cvu),DBCA,NETCA

  • How do we verify that RAC instances are running?

Issue the following query from any one node connecting through SQL*PLUS.
$connect sys/sys as sysdba
The query gives the Instance number under INST_NUMBER column, Host: Instance name under INST_NAME column.

  • What is FAN?

Fast application Notification as it abbreviates to FAN relates to the events related to instances, services and nodes. This is a notification mechanism that Oracle RAC uses to notify other processes about the configuration and service level information that includes service status changes such as, UP or DOWN events. Applications can respond to FAN events and take immediate action.

  • Where can we apply FAN UP and DOWN events?

FAN UP and FAN DOWN events can be applied to instances, services and nodes.
State the use of FAN events in case of a cluster configuration change?
During times of cluster configuration changes, Oracle RAC high availability framework publishes a FAN event immediately when a state change occurs in the cluster. So applications can receive FAN events and react immediately. This prevents applications from polling database and detecting a problem after such a state change.

  • Why should we have separate homes for ASM instance?

It is a good practice to have ASM home separate from the database home (ORACLE_HOME).This helps in upgrading and patching ASM and the Oracle database software independent of each other. Also, we can deinstall the Oracle database software independent of the ASM instance.

  • What is the advantage of using ASM?

Having ASM is the Oracle recommended storage option for RAC databases as the ASM maximizes performance by managing the storage configuration across the disks. ASM does this by distributing the database file across all of the available storage within our cluster database environment.

  • What is rolling upgrade?

It is a new ASM feature from Database 11g. ASM instances in Oracle database 11g release(from 11.1) can be upgraded or patched using rolling upgrade feature. This enables us to patch or upgrade ASM nodes in a clustered environment without affecting database availability. During a rolling upgrade we can maintain a functional cluster while one or more of the nodes in the cluster are running in different software versions.

  • Can rolling upgrade be used to upgrade from 10g to 11g database?

No,it can be used only for Oracle database 11g releases(from 11.1).

  • State the initialization parameters that must have same value for every instance in an Oracle RAC database:

Some initialization parameters are critical at the database creation time and must have same values. Their value must be specified in SPFILE or PFILE for every instance. The list of parameters that must be identical on every instance are given below:

  • What is ORA-00603: ORACLE server session terminated by fatal error or ORA-29702: error occurred in Cluster Group Service operation?

RAC node name was listed in the loopback address…

  • Can the DML_LOCKS and RESULT_CACHE_MAX_SIZE be identical on all instances?

These parameters can be identical on all instances only if these parameter values are set to zero.
What two parameters must be set at the time of starting up an ASM instance in a RAC environment? The parameters CLUSTER_DATABASE and INSTANCE_TYPE must be set.

  • Mention the components of Oracle clusterware:

Oracle clusterware is made up of components like voting disk and Oracle Cluster Registry(OCR).

  • What is a CRS resource?

Oracle clusterware is used to manage high-availability operations in a cluster. Anything that Oracle Clusterware manages is known as a CRS resource. Some examples of CRS resources are database, an instance, a service, a listener, a VIP address, an application process etc.

  • What is the use of OCR?

Oracle clusterware manages CRS resources based on the configuration information of CRS resources stored in OCR(Oracle Cluster Registry).

  • How does a Oracle Clusterware manage CRS resources?

Oracle clusterware manages CRS resources based on the configuration information of CRS resources stored in OCR(Oracle Cluster Registry).

  • Name some Oracle clusterware tools and their uses?

OIFCFG – allocating and deallocating network interfaces
OCRCONFIG – Command-line tool for managing Oracle Cluster Registry
OCRDUMP – Identify the interconnect being used
CVU – Cluster verification utility to get status of CRS resources

  • What are the modes of deleting instances from Oracle Real Application cluster Databases?

We can delete instances using silent mode or interactive mode using DBCA(Database Configuration Assistant).

  • How do we remove ASM from a Oracle RAC environment?

We need to stop and delete the instance in the node first in interactive or silent mode. After that asm can be removed using srvctl tool as follows:
srvctl stop asm -n node_name
srvctl remove asm -n node_name
We can verify if ASM has been removed by issuing the following command:
srvctl config asm -n node_name

  • How do we verify that an instance has been removed from OCR after deleting an instance?

Issue the following srvctl command:
srvctl config database -d database_name
cd CRS_HOME/bin

  • How do we verify an existing current backup of OCR?

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

  • What are the performance views in an Oracle RAC environment?

We have v$ views that are instance specific. In addition we have GV$ views called as global views that has an INST_ID column of numeric data type. GV$ views obtain information from individual V$ views.

  • What are the types of connection load-balancing?

There are two types of connection load-balancing: server-side load balancing and client-side load balancing.

  • What is the difference between server-side and client-side connection load balancing?

Client-side balancing happens at client side where load balancing is done using listener. In case of server-side load balancing listener uses a load-balancing advisory to redirect connections to the instance providing best service.

  • What are the three greatest benefits that RAC provides??

The three main benefits are availability, scalability, and the ability to use low cost commodity hardware. RAC allows an application to scale vertically, by adding CPU, disk and memory resources to an individual server. But RAC also provides horizontal scalability, which is achieved by adding new nodes into the cluster. RAC also allows an organization to bring these resources online as they are needed. This can save a small or midsize organization a lot of money in the early stages of a project.
In a RAC environment, if a node in the cluster fails, the application continues to run on the surviving nodes contained in the cluster. If your application is configured correctly, most users won’t even know that the node they were running on became unavailable.

  • Oracle RAC Background processes

1. Lock Monitor Processes ( LMON)
2. Lock Monitor Services (LMS)
3. Lock Monitor Daemon Process ( LMD)
4. LCKn ( Lock Process)
5. DIAG (Diagnostic Daemon)


1. Lock Monitor Processes ( LMON)

It Maintains GCS memory structures.Handles the abnormal termination of processes and instances. Reconfiguration of locks & resources when an instance joins or leaves the cluster are handled by LMON ( During reconfiguration LMON generate the trace files). It responsible for executing dynamic lock remastering every 10 mins ( Only in 10g R2 & later versions).
LMON Processes manages the global locks & resources. It monitors all instances in cluster, primary for dictionary cache locks,library cache locks & deadlocks on deadlock sensitive on enqueue & resources. LMON also provides cluster group services. Also called Global enqueue service monitor.

2. Lock Monitor Services (LMS)
LMS is most very active background processes.
Consuming significant amount of CPU time. ( 10g R2 – ensure that LMS process does not encounter the CPU starvation).
Its primary job is to transport blocks across the nodes for cache-fusion requests.
If there is a consistent-read request, the LMS process rolls back the block, makes a Consistent-Read image of the block and then ship this block across the HSI (High Speed Interconnect) to the process requesting from a remote node.
LMS must also check constantly with the LMD background process (or our GES process) to get the lock requests placed by the LMD process. Each node have 2 or more LMS processes.
GCS_SERVER_PROCESSES –> no of LMS processes specified in init. ora parameter.
Above parameter value set based on number of cpu’s ( MIN(CPU_COUNT/2,2))
10gR2, single CPU instance,only one LMS processes started.
Increasing the parameter value,if global cache activity is very high.
Also called the GCS (Global Cache Services) processes.
Internal View: X$KJMSDP

3. Lock Monitor Daemon Process ( LMDn)
LMD process performs global lock deadlock detection.
Also monitors for lock conversion timeouts.
Also sometimes referred to as the GES (Global Enqueue Service) daemon since its job is to manage the global enqueue and global resource access.
LMD process also handles deadlock detection and remote enqueue requests.
Remote resource requests are the requests originating from another instance.
Internal View: X$KJMDDP

4. LCKn ( Lock Process)

Manages instance resource requests & cross instance calls for shared resources.
During instance recovery,it builds a list of invalid lock elements and validates lock elements.

5. DIAG (Diagnostic Daemon)

Oracle 10g – this one new background processes ( New enhanced diagnosability framework).
Regularly monitors the health of the instance.
Also checks instance hangs & deadlocks.
It captures the vital diagnostics data for instance & process failures.

  • What is Split brain syndrome in Oracle RAC?

It occurs when the instance members in a RAC fail to ping/connect to each other via this private interconnect, but the servers are all pysically up and running and the database instance on each of these servers is also running. These individual nodes are running fine and can conceptually accept user connections and work independently. So basically due to lack of commincation the instance thinks that the other instance that it is not able to connect is down and it needs to do something about the situation. The problem is if we leave these instance running, the sane block might get read, updated in these individual instances and there would be data integrity issue, as the blocks changed in one instance, will not be locked and could be over-written by another instance. Oracle has efficiently implemented check for the split brain syndrome.

Split brain condition

In RAC if any node becomes inactive, or if other nodes are unable to ping/connect to a node in the RAC, then the node which first detects that one of the node is not accessible, it will evict that node from the RAC group. e.g. there are 4 nodes in a rac instance, and node 3 becomes unavailable, and node 1 tries to connect to node 3 and finds it not responding, then node 1 will evict node 3 out of the RAC groups and will leave only Node1, Node2 & Node4 in the RAC group to continue functioning.
The split brain concepts can become more complicated in large RAC setups. For example there are 10 RAC nodes in a cluster. And say 4 nodes are not able to communicate with the other 6. So there are 2 groups formed in this 10 node RAC cluster ( one group of 4 nodes and other of 6 nodes). Now the nodes will quickly try to affirm their membership by locking controlfile, then the node that lock the controlfile will try to check the votes of the other nodes. The group with the most number of active nodes gets the preference and the others are evicted. Moreover, I have seen this node eviction issue with only 1 node getting evicted and the rest function fine, so I cannot really testify that if thats how it work by experience, but this is the theory behind it.
When we see that the node is evicted, usually oracle rac will reboot that node and try to do a cluster reconfiguration to include back the evicted node.
You will see oracle error: ORA-29740, when there is a node eviction in RAC. There are many reasons for a node eviction like heart beat not received by the controlfile, unable to communicate with the clusterware etc.

And also You can go through Metalink Note ID: 219361.1

  • Is there a defined rule that RAC clusters “should” be an odd number because of this scenario?
    i.e. if you have a 4 node RAC cluster and 2 nodes simultaneously die, will this cause the other 2 nodes to evict each other / terminate?

Based on this rule, 50% of the RAC cluster (or one of the 2 groups) has lost contact with the other and therefore yields a similar scenario to a 2 node cluster. No, The number of RAC nodes is unimportant. If you have a split brain (interconnect is lost) the nodes will still be able to communicate via. the voting disks (disk heartbeat). Via. this it is always possible to tell if a node has to leave the cluster (for this reason a node evicts if it cannot access the majority of the voting disks).

However there are 2 eviction scenarios. If you have a split brain in an unequal way, the bigger subcluster will survive.
In case of an equal split brain situation the subcluster containing the node which has the OCR master role at that moment will survive. Since if the cluster evicts the node with the OCR master role, this role would have to be additionally relocated – and it is easier to evict the other nodes not having that role.

So if in your scenario 2 nodes will simultaneous die, this will not impact the running nodes, since they will detect these 2 nodes are not running anymore. This is one of the scenario where both heartbeats (disks and interconnect) is needed to detect that.

Fariddin Shaikh:

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