RAC Architecture

Oracle Real Application clusters allows multiple instances to access a single database, the instances will be running on multiple nodes. In an standard Oracle configuration a database can only be mounted by one instance but in a RAC environment many instances can access a single database.

Oracle's RAC is heavy dependent on a efficient, high reliable high speed private network called the interconnect, make sure when designing a RAC system that you get the best that you can afford.

The table below describes the difference of a standard oracle database (single instance) an a RAC environment

ComponentSingle Instance Environment

RAC Environment

SGA Instance has its own SGA Each instance has its own SGA

Background processes

Instance has its own set of background processes

Each instance has its own set of background processes

DatafilesAccessed by only one instance

Shared by all instances (shared storage)

Control Files Accessed by only one instance

Shared by all instances (shared storage)

Online Redo Logfile Dedicated for write/read to only one instance

Only one instance can write but other instances can read during recovery and archiving. If an instance is shutdown, log switches by other instances can force the idle instance redo logs to be archived

Archived Redo Logfile

Dedicated to the instance Private to the instance but other instances will need access to all required archive logs during media recovery

Flash Recovery Log Accessed by only one instance

Shared by all instances (shared storage)

Alert Log and Trace Files

Dedicated to the instance Private to each instance, other instances never read or write to those files.

ORACLE_HOME Multiple instances on the Same as single instance plus can be placed

same server accessing different databases ca use the same executable files

on shared file system allowing a common ORACLE_HOME for all instances in a RAC environment.

RAC Components

The major components of a Oracle RAC system are

Shared disk system Oracle Clusterware

Cluster Interconnects

Oracle Kernel Components

The below diagram describes the basic architecture of the Oracle RAC environment

Here are a list of processes running on a freshly installed RAC

Disk architecture

With today's SAN and NAS disk storage systems, sharing storage is fairly easy and is required for a RAC environment, you can use the below storage setups

SAN (Storage Area Networks) - generally using fibre to connect to the SAN NAS ( Network Attached Storage) - generally using a network to connect to the NAS

using either NFS, ISCSI

JBOD - direct attached storage, the old traditional way and still used by many companies as a cheap option

All of the above solutions can offer multi-pathing to reduce SPOFs within the RAC environment, there is no reason not to configure multi-pathing as the cost is cheap when adding additional paths to the disk because most of the expense is paid when out when configuring the first path, so an additional controller card and network/fibre cables is all that is need.

The last thing to think about is how to setup the underlining disk structure this is known as a raid level, there are about 12 different raid levels that I know off, here are the most common ones

raid 0 (Striping)

A number of disks are concatenated together to give the appearance of one very large disk.

Advantages   Improved performance   Can Create very large Volumes

Disadvantages   Not highly available (if one disk fails, the volume fails)

raid 1 (Mirroring)

A single disk is mirrored by another disk, if one disk fails the system is unaffected as it can use its mirror.

Advantages   Improved performance   Highly Available (if one disk fails the mirror takes over)

Disadvantages   Expensive (requires double the number of disks)

raid 5

Raid stands for Redundant Array of Inexpensive Disks, the disks are striped with parity across 3 or more disks, the parity is used in the event that one of the disks fails, the data on the failed disk is reconstructed by using the parity bit.

Advantages   Improved performance (read only)   Not expensive

Disadvantages   Slow write operations (caused by having to create the parity bit)

There are many other raid levels that can be used with a particular hardware environment for example EMC storage uses the RAID-S, HP storage uses Auto RAID, so check with the

manufacture for the best solution that will provide you with the best performance and resilience.

Once you have you storage attached to the servers, you have three choices on how to setup the disks

Raw Volumes - normally used for performance benefits, however they are hard to manage and backup

Cluster FileSystem - used to hold all the Oracle datafiles can be used by windows and linux, its not used widely

Automatic Storage Management (ASM) - Oracle choice of storage management, its a portable, dedicated and optimized cluster filesystem

I will only be discussing ASM, which i have already have a topic on called Automatic Storage Management.

Oracle Clusterware

Oracle Clusterware software is designed to run Oracle in a cluster mode, it can support you to 64 nodes, it can even be used with a vendor cluster like Sun Cluster.

The Clusterware software allows nodes to communicate with each other and forms the cluster that makes the nodes work as a single logical server. The software is run by the Cluster Ready Services (CRS) using the Oracle Cluster Registry (OCR) that records and maintains the cluster and node membership information and the voting disk which acts as a tiebreaker during communication failures. Consistent heartbeat information travels across the interconnect to the voting disk when the cluster is running.

The CRS has four components

OPROCd - Process Monitor Daemon CRSd - CRS daemon, the failure of this daemon results in a node being reboot to

avoid data corruption

OCSSd - Oracle Cluster Synchronization Service Daemon (updates the registry)

EVMd - Event Volume Manager Daemon

The OPROCd daemon provides the I/O fencing for the Oracle cluster, it uses the hangcheck timer or watchdog timer for the cluster integrity. It is locked into memory and runs as a realtime processes, failure of this daemon results in the node being rebooted. Fencing is used to protect the data, if a node were to have problems fencing presumes the worst and protects the data thus restarts the node in question, its better to be save than sorry.

The CRSd process manages resources such as starting and stopping the services and failover of the application resources, it also spawns separate processes to manage application resources. CRS manages the OCR and stores the current know state of the cluster, it requires a public, private and VIP interface in order to run.

OCSSd provides synchronization services among nodes, it provides access to the node membership and enables basic cluster services, including cluster group services and locking, failure of this daemon causes the node to be rebooted to avoid split-brain situations.

The below functions are covered by the OCSSd

CSS provides basic Group Services Support, it is a distributed group membership system that allows applications to coordinate activities to archive a common result.

Group services use vendor clusterware group services when it is available.

Lock services provide the basic cluster-wide serialization locking functions, it uses the First In, First Out (FIFO) mechanism to manage locking

Node services uses OCR to store data and updates the information during reconfiguration, it also manages the OCR data which is static otherwise.

The last component is the Event Management Logger, which runs the EVMd process. The daemon spawns a processes called evmlogger and generates the events when things happen. The evmlogger spawns new children processes on demand and scans the callout directory to invoke callouts. Death of the EVMd daemon will not halt the instance and will be restarted.

Quick recap

CRS Process Functionality Failure of the Process Run AS

OPROCd - Process Monitor

provides basic cluster integrity services

Node Restart root

EVMd - Event Management

spawns a child process event logger and generates callouts

Daemon automatically restarted, no node restart

oracle

OCSSd - Cluster Synchronization Services

basic node membership, group services, basic locking

Node Restart oracle

CRSd - Cluster Ready Services

resource monitoring, failover and node recovery

Daemon restarted automatically, no node restart

root

The cluster-ready services (CRS) is a new component in 10g RAC, its is installed in a separate home directory called ORACLE_CRS_HOME. It is a mandatory component but can be used with a third party cluster (Veritas, Sun Cluster), by default it manages the node membership functionality along with managing regular RAC-related resources and services

RAC uses a membership scheme, thus any node wanting to join the cluster as to become a member. RAC can evict any member that it seems as a problem, its primary concern is protecting the data. You can add and remove nodes from the cluster and the membership increases or decrease, when network problems occur membership becomes the deciding factor on which part stays as the cluster and what nodes get evicted, the use of a voting disk is used which I will talk about later.

The resource management framework manage the resources to the cluster (disks, volumes), thus you can have only have one resource management framework per resource. Multiple frameworks are not supported as it can lead to undesirable affects.

The Oracle Cluster Ready Services (CRS) uses the registry to keep the cluster configuration, it should reside on a shared storage and accessible to all nodes within the cluster. This shared storage is known as the Oracle Cluster Registry (OCR) and its a major part of the cluster, it is automatically backed up (every 4 hours) the daemons plus you can manually back it up. The OCSSd uses the OCR extensively and writes the changes to the registry

The OCR keeps details of all resources and services, it stores name and value pairs of information such as resources that are used to manage the resource equivalents by the CRS stack. Resources with the CRS stack are components that are managed by CRS and have the information on the good/bad state and the callout scripts. The OCR is also used to supply bootstrap information ports, nodes, etc, it is a binary file.

The OCR is loaded as cache on each node, each node will update the cache then only one node is allowed to write the cache to the OCR file, the node is called the master. The Enterprise manager also uses the OCR cache, it should be at least 100MB in size. The CRS daemon will update the OCR about status of the nodes in the cluster during reconfigurations and failures.

The voting disk (or quorum disk) is shared by all nodes within the cluster, information about the cluster is constantly being written to the disk, this is known as the heartbeat. If for any reason a node cannot access the voting disk it is immediately evicted from the cluster, this protects the cluster from split-brains (the Instance Membership Recovery algorithm IMR is used to detect and resolve split-brains) as the voting disk decides what part is the really cluster. The voting disk manages the cluster membership and arbitrates the cluster ownership during communication failures between nodes. Voting is often confused with quorum the are similar but distinct, below details what each means

VotingA vote is usually a formal expression of opinion or will in response to a proposed decision

Quorumis defined as the number, usually a majority of members of a body, that, when assembled is legally competent to transact business

The only vote that counts is the quorum member vote, the quorum member vote defines the cluster. If a node or group of nodes cannot archive a quorum, they should not start any services because they risk conflicting with an established quorum.

The voting disk has to reside on shared storage, it is a a small file (20MB) that can be accessed by all nodes in the cluster. In Oracle 10g R1 you can have only one voting disk, but in R2 you can have upto 32 voting disks allowing you to eliminate any SPOF's.

The original Virtual IP in Oracle was Transparent Application Failover (TAF), this had limitations, this has now been replaced with cluster VIPs. The cluster VIPs will failover to working nodes if a node should fail, these public IPs are configured in DNS so that users can access them. The cluster VIPs are different from the cluster interconnect IP address and are only used to access the database.

The cluster interconnect is used to synchronize the resources of the RAC cluster, and also used to transfer some data from one instance to another. This interconnect should be private, highly available and fast with low latency, ideally they should be on a minimum private 1GB network. What ever hardware you are using the NIC should use multi-pathing (Linux - bonding, Solaris - IPMP). You can use crossover cables in a QA/DEV environment but it is not supported in a production environment, also crossover cables limit you to a two node cluster.

Oracle Kernel Components

The kernel components relate to the background processes, buffer cache and shared pool and managing the resources without conflicts and corruptions requires special handling.

In RAC as more than one instance is accessing the resource, the instances require better coordination at the resource management level. Each node will have its own set of buffers but will be able to request and receive data blocks currently held in another instance's cache. The management of data sharing and exchange is done by the Global Cache Services (GCS).

All the resources in the cluster group form a central repository called the Global Resource Directory (GRD), which is distributed. Each instance masters some set of resources and together all instances form the GRD. The resources are equally distributed among the nodes based on their weight. The GRD is managed by two services called Global Caches Services (GCS) and Global Enqueue Services (GES), together they form and manage the GRD. When a node leaves the cluster, the GRD portion of that instance needs to be redistributed to the surviving nodes, a similar action is performed when a new node joins.

RAC Background Processes

Each node has its own background processes and memory structures, there are additional processes than the norm to manage the shared resources, theses additional processes maintain cache coherency across the nodes.

Cache coherency is the technique of keeping multiple copies of a buffer consistent between different Oracle instances on different nodes. Global cache management ensures that access to a master copy of a data block in one buffer cache is coordinated with the copy of the block in another buffer cache.

The sequence of a operation would go as below

1. When instance A needs a block of data to modify, it reads the bock from disk, before reading it must inform the GCS (DLM). GCS keeps track of the lock status of the data block by keeping an exclusive lock on it on behalf of instance A

2. Now instance B wants to modify that same data block, it to must inform GCS, GCS will then request instance A to release the lock, thus GCS ensures that instance B gets the latest version of the data block (including instance A modifications) and then exclusively locks it on instance B behalf.

3. At any one point in time, only one instance has the current copy of the block, thus keeping the integrity of the block.

GCS maintains data coherency and coordination by keeping track of all lock status of each block that can be read/written to by any nodes in the RAC. GCS is an in memory database that contains information about current locks on blocks and instances waiting to acquire locks. This is known as Parallel Cache Management (PCM). The Global Resource Manager (GRM) helps to coordinate and communicate the lock requests from Oracle processes between instances in the RAC. Each instance has a buffer cache in its SGA, to ensure that each RAC instance obtains the block that it needs to satisfy a query or transaction. RAC uses two processes the GCS and GES which maintain records of lock status of each data file and each cached block using a GRD.

So what is a resource, it is an identifiable entity, it basically has a name or a reference, it can be a area in memory, a disk file or an abstract entity. A resource can be owned or locked in various states (exclusive or shared). Any shared resource is lockable and if it is not shared no access conflict will occur.

A global resource is a resource that is visible to all the nodes within the cluster. Data buffer cache blocks are the most obvious and most heavily global resource, transaction enqueue's and database data structures are other examples. GCS handle data buffer cache blocks and GES handle all the non-data block resources.

All caches in the SGA are either global or local, dictionary and buffer caches are global, large and java pool buffer caches are local. Cache fusion is used to read the data buffer cache from another instance instead of getting the block from disk, thus cache fusion moves current copies of data blocks between instances (hence why you need a fast private network), GCS manages the block transfers between the instances.

Finally we get to the processes

Oracle RAC Daemons and Processes

LMSn Lock Manager Server process - GCS

this is the cache fusion part and the most active process, it handles the consistent copies of blocks that are transferred between instances. It receives requests from LMD to perform lock requests. I rolls back any uncommitted transactions. There can be up to ten LMS processes running and can be started dynamically if demand requires it.

they manage lock manager service requests for GCS resources and send them to a service queue to be handled by the LMSn process. It also handles global deadlock detection and monitors for lock conversion timeouts.

as a performance gain you can increase this process priority to make sure CPU starvation does not occur

you can see the statistics of this daemon by looking at the view X$KJMSDP

LMONLock Monitor Process - GES

this process manages the GES, it maintains consistency of GCS memory structure in case of process death. It is also responsible for cluster reconfiguration and locks reconfiguration (node joining or leaving), it checks for instance deaths and listens for local messaging.

A detailed log file is created that tracks any reconfigurations that have happened.

LMD

Lock Manager Daemon - GES

this manages the enqueue manager service requests for the GCS. It also handles deadlock detention and remote resource requests from other instances.

you can see the statistics of this daemon by looking at the view X$KJMDDP

LCK0Lock Process - GES

manages instance resource requests and cross-instance call operations for shared resources. It builds a list of invalid lock elements and validates lock elements during recovery.

DIAGDiagnostic Daemon

This is a lightweight process, it uses the DIAG framework to monitor the health of the cluster. It captures information for later diagnosis in the event of failures. It will perform any necessary recovery if an operational hang is detected.

RAC Administration

I am only going to talk about RAC administration, if you need Oracle administration then see my Oracle section.

It is recommended that the spfile (binary parameter file) is shared between all nodes within the cluster, but it is possible that each instance can have its own spfile. The parameters can be grouped into three categories

Unique parameters These parameters are unique to each instance, examples would be instance_nameundo_tablespace

Identical parameters Parameters in this category must be the same for each instance, examples would be control_file

Neither unique or identical parameters

parameters that are not in any of the above, examples would be db_cache_size, large_pool_size, local_listener and gcs_servers_processes

The main unique parameters that you should know about are

instance_name - defines the name of the Oracle instance (default is the value of the oracle_sid variable)

instance_number - a unique number for each instance must be greater than 0 but smaller than the max_instance parameter

thread - specifies the set of redolog files to be used by the instance

undo_tablespace - specifies the name of the undo tablespace to be used by the instance

rollback_segments - you should use Automatic Undo Management

cluster_interconnects - use if only if Oracle has trouble not picking the correct interconnects

The identical unique parameters that you should know about are below you can use the below query to view all of them

       select name, isinstance_modifiable from v$parameter where isinstance_modifiable = 'false' order by name;

cluster_database - options are true or false, mounts the control file in either share (cluster) or exclusive mode, use false in the below cases

o Converting from no archive log mode to archive log mode and vice versa

o Enabling the flashback database feature

o Performing a media recovery on a system table

o Maintenance of a node

active_instance_count - used for primary/secondary RAC environments

cluster_database_instances - specifies the number of instances that will be accessing the database (set to maximum # of nodes)

dml_locks - specifies the number of DML locks for a particular instance (only change if you get ORA-00055 errors)

gc_files_to_locks - specify the number of global locks to a data file, changing this disables the Cache Fusion.

max_commit_propagation_delay - influences the mechanism Oracle uses to synchronize the SCN among all instances

instance_groups - specify multiple parallel query execution groups and assigns the current instance to those groups

parallel_instance_group - specifies the group of instances to be used for parallel query execution

gcs_server_processes - specify the number of lock manager server (LMS) background processes used by the instance for Cache Fusion

remote_listener - register the instance with listeners on remote nodes.

syntax for parameter file

<instance_name>.<parameter_name>=<parameter_value>

inst1.db_cache_size = 1000000*.undo_management=auto

Examplealter system set db_2k_cache_size=10m scope=spfile sid='inst1';

Note: use the sid option to specify a particular instance

Starting and Stopping Instances

The srvctl command is used to start/stop an instance, you can also use sqlplus to start and stop the instance

start all instances

srvctl start database -d <database> -o <option>

Note: starts listeners if not already running, you can use the -o option to specify startup/shutdown options, see below for options

forceopenmountnomount

stop all instances

srvctl stop database -d <database> -o <option>

Note: the listeners are not stopped, you can use the -o option to specify startup/shutdown options, see below for options

immediateabort normaltransactional

start/stop particular instance

srvctl [start|stop] database -d <database> -i <instance>,<instance>

Undo Management

To recap on undo management you can see my undo section, instances in a RAC do not share undo, they each have a dedicated undo tablespace. Using the undo_tablespace parameter each instance can point to its own undo tablespace

undo tablespace

instance1.undo_tablespace=undo_tbs1instance2.undo_tablespace=undo_tbs2

With todays Oracle you should be using automatic undo management, again I have a detailed discussion on AUM in my undo section.

Temporary Tablespace

I have already discussed temporary tablespace's, in a RAC environment you should setup a temporary tablespace group, this group is then used by all instances of the RAC. Each instance creates a temporary segment in the temporary tablespace it is using. If an instance is running a large sort, temporary segments can be reclaimed from segments from other instances in that tablespace.

useful views

gv$sort_segment - explore current and maximum sort segment usage statistics (check columns freed_extents, free_requests ,if they grow increase tablespace size) gv$tempseg_usage - explore temporary segment usage details such as name, SQL, etc v$tempfile - identify - temporary datafiles being used for the temporary tablespace

Redologs

I have already discussed redologs, in a RAC environment every instance has its own set of redologs. Each instance has exclusive write access to its own redologs, but each instance can read each others redologs, this is used for recovery. Redologs are located on the shared storage so that all instances can have access to each others redologs. The process is a little different to the standard Oracle when changing the archive mode

archive mode (RAC)

SQL> alter system set cluster_database=false scope=spfile sid='prod1';srvctl stop database -d <database>SQL> startup mountSQL> alter database archivelog;SQL> alter system set cluster_database=true scope=spfile sid='prod1';SQL> shutdown;srvctl start database -d prod

Flashback

Again I have already talked about flashback, there is no difference in RAC environment apart from the setting up

flashback (RAC)

## Make sure that the database is running in archive log mode SQL> archive log list

## Setup the flashbackSQL> alter system set cluster_database=false scope=spfile sid='prod1';SQL> alter system set DB_RECOVERY_FILE_DEST_SIZE=200M scope=spfile;SQL> alter system set DB_RECOVERY_FILE_DEST='/ocfs2/flashback' scope=spfile;srvctl stop database -p prod1SQL> startup mountSQL> alter database flashback on;

SQL> shutdown;srvctl start database -p prod1

SRVCTL command

We have already come across the srvctl above, this command is called the server control utility. It can divided into two categories

Database configuration tasks Database instance control tasks

Oracle stores database configuration in a repository, the configuration is stored in the Oracle Cluster Registry (OCR) that was created when RAC was installed, it will be located on the shared storage. Srvctl uses CRS to communicate and perform startup and shutdown commands on other nodes.

I suggest that you lookup the command but I will provide a few examples

display the registered databases

srvctl config database

status

srvctl status database -d <databasesrvctl status instance -d <database> -i <instance> srvctl status nodeapps -n <node>srvctl status service -d <database> srvctl status asm -n <node>

stopping/starting

srvctl stop database -d <database>srvctl stop instance -d <database> -i <instance>,<instance>srvctl stop service -d <database> [-s <service><service>] [-i <instance>,<instance>]srvctl stop nodeapps -n <node>srvctl stop asm -n <node>

srvctl start database -d <database>srvctl start instance -d <database> -i <instance>,<instance>srvctl start service -d <database> -s <service><service> -i <instance>,<instance>srvctl start nodeapps -n <node>srvctl start asm -n <node>

adding/removing srvctl add database -d <database> -o <oracle_home>srvctl add instance -d <database> -i <instance> -n <node>srvctl add service -d <database> -s <service> -r <preferred_list>srvctl add nodeapps -n <node> -o <oracle_home> -A <name|ip>/networksrvctl add asm -n <node> -i <asm_instance> -o <oracle_home>

srvctl remove database -d <database> -o <oracle_home>srvctl remove instance -d <database> -i <instance> -n <node>srvctl remove service -d <database> -s <service> -r <preferred_list>

srvctl remove nodeapps -n <node> -o <oracle_home> -A <name|ip>/networksrvctl asm remove -n <node>

Services

Services are used to manage the workload in Oracle RAC, the important features of services are

used to distribute the workload can be configured to provide high availability

provide a transparent way to direct workload

The view v$services contains information about services that have been started on that instance, here is a list from a fresh RAC installation

The table above is described below

Goal - allows you to define a service goal using service time, throughput or none Connect Time Load Balancing Goal - listeners and mid-tier servers contain current

information about service performance

Distributed Transaction Processing - used for distributed transactions

AQ_HA_Notifications - information about nodes being up or down will be sent to mid-tier servers via the advance queuing mechanism

Preferred and Available Instances - the preferred instances for a service, available ones are the backup instances

You can administer services using the following tools

DBCA EM (Enterprise Manager)

DBMS_SERVICES

Server Control (srvctl)

Two services are created when the database is first installed, these services are running all the time and cannot be disabled.

sys$background - used by an instance's background processes only

sys$users - when users connect to the database without specifying a service they use this service

add

srvctl add service -d D01 -s BATCH_SERVICE -r node1,node2 -a node3

Note: the options are describe below

-d - database-s - the service-r - the service will running on the these nodes-a - if nodes in the -r list are not running then run on this node

remove srvctl remove service -d D01 -s BATCH_SERVICE

start srvctl start service -d D01 -s BATCH_SERVICE

stop srvctl stop service -d D01 -s BATCH_SERVICE

status srvctl status service -d D10 -s BATCH_SERVICE

service (example)

## create the JOB class BEGIN  DBMS_SCHEDULER.create_job_class(     job_class_name => 'BATCH_JOB_CLASS',     service            => 'BATCH_SERVICE');END;/

## Grant the privileges to execute the job grant execute on sys.batch_job_class to vallep;

## create a job associated with a job class BEGIN  DBMS_SCHDULER.create_job(    job_name => 'my_user.batch_job_test',   job_type => 'PLSQL_BLOCK',    job_action => SYSTIMESTAMP'    repeat_interval => 'FREQ=DAILY;',    job_class => 'SYS.BATCH_JOB_CLASS',    end_date => NULL,    enabled => TRUE,    comments => 'Test batch job to show RAC services');END;/

## assign a job class to an existing job exec dbms_scheduler.set_attribute('MY_BATCH_JOB', 'JOB_CLASS', 'BATCH_JOB_CLASS');

Cluster Ready Services (CRS)

CRS is Oracle's clusterware software, you can use it with other third-party clusterware software, though it is not required (apart from HP True64).

CRS is start automatically when the server starts, you should only stop this service in the following situations

Applying a patch set to $ORA_CRS_HOME O/S maintenance

Debugging CRS problems

CRS Administration

Starting

## Starting CRS using Oracle 10g R1not possible

## Starting CRS using Oracle 10g R2$ORA_CRS_HOME/bin/crsctl start crs

Stopping

## Stopping CRS using Oracle 10g R1 srvctl stop -d database <database>srvctl stop asm -n <node>srvctl stop nodeapps -n <node>/etc/init.d/init.crs stop

## Stopping CRS using Oracle 10g R2 $ORA_CRS_HOME/bin/crsctl stop crs

disabling/enabling

## stop CRS restarting after a reboot, basically permanent over reboots

## Oracle 10g R1 /etc/init.d/init.crs [disable|enable]

## Oracle 10g R2$ORA_CRS_HOME/bin/crsctl [disable|enable] crs

Checking

$ORA_CRS_HOME/bin/crsctl check crs$ORA_CRS_HOME/bin/crsctl check evmd$ORA_CRS_HOME/bin/crsctl check cssd$ORA_CRS_HOME/bin/crsctl check crsd$ORA_CRS_HOME/bin/crsctl check install -wait 600

Resource Applications (CRS Utilities)

Status

$ORA_CRS_HOME/bin/crs_stat$ORA_CRS_HOME/bin/crs_stat -t $ORA_CRS_HOME/bin/crs_stat -ls $ORA_CRS_HOME/bin/crs_stat -p

Note:-t more readable display -ls permission listing-p parameters

create profile $ORA_CRS_HOME/bin/crs_profile

register/unregister $ORA_CRS_HOME/bin/crs_register

application $ORA_CRS_HOME/bin/crs_unregister

Start/Stop an application $ORA_CRS_HOME/bin/crs_start$ORA_CRS_HOME/bin/crs_stop

Resource permissions $ORA_CRS_HOME/bin/crs_getparam$ORA_CRS_HOME/bin/crs_setparam

Relocate a resource $ORA_CRS_HOME/bin/crs_relocate

Nodes

member number/name

olsnodes -n

Note: the olsnodes command is located in $ORA_CRS_HOME/bin

local node name olsnodes -l

activates logging olsnodes -g

Oracle Interfaces

Display oifcfg getif

Delete oicfg delig -global

Setoicfg setif -global <interface name>/<subnet>:publicoicfg setif -global <interface name>/<subnet>:cluster_interconnect

Global Services Daemon Control

Starting gsdctl start

Stopping gsdctl stop

Status gsdctl status

Cluster Configuration (clscfg is used during installation)

create a new configuration

clscfg -install

Note: the clscfg command is located in $ORA_CRS_HOME/bin

upgrade or downgrade and existing configuration

clscfg -upgradeclscfg –downgrade

add or delete a node from the configuration

clscfg -addclscfg –delete

create a special single-node configuration for ASM

clscfg –local

brief listing of terminology used in the other nodes

clscfg –concepts

used for tracing clscfg –trace

Help clscfg -h

Cluster Name Check

print cluster name cemutlo -n

Note: in Oracle 9i the ulity was called "cemutls", the command is located in $ORA_CRS_HOME/bin

print the clusterware version cemutlo -w

Note: in Oracle 9i the ulity was called "cemutls"

Node Scripts

Add Node addnode.sh

Note: see adding and deleting nodes

Delete Node deletenode.sh

Note: see adding and deleting nodes

Oracle Cluster Registry (OCR)

As you already know the OCR is the registry that contains information

Node list Node membership mapping

Database instance, node and other mapping information

Characteristics of any third-party applications controlled by CRS

The file location is specified during the installation, the file pointer indicating the OCR device location is the ocr.loc, this can be in either of the following

linux - /etc/oracle solaris - /var/opt/oracle

The file contents look something like below, this was taken from my installation

orc.lococrconfig_loc=/u02/oradata/racdb/OCRFileocrmirrorconfig_loc=/u02/oradata/racdb/OCRFile_mirrorlocal_only=FALSE

OCR is import to the RAC environment and any problems must be immediately actioned, the command can be found in located in $ORA_CRS_HOME/bin

OCR Utilities

log file $ORA_HOME/log/<hostname>/client/ocrconfig_<pid>.log

checking

ocrcheck

Note: will return the OCR version, total space allocated, space used, free space, location of each device and the result of the integrity check

dump contents ocrdump

Note: by default it dumps the contents into a file named OCRDUMPFILE in the current directory

export/importocrconfig -export <file>

ocrconfig -restore <file>

backup/restore

# show backupsocrconfig -showbackup

# to change the location of the backup, you can even specify a ASM disk

ocrconfig -backuploc <path|+asm>

# perform a backup, will use the location specified by the -backuploc location ocrconfig -manualbackup

# perform a restoreocrconfig -restore <file>

# delete a backuporcconfig -delete <file>

Note: there are many more option so see the ocrconfig man page

add/remove/replace

## add/relocate the ocrmirror file to the specified location ocrconfig -replace ocrmirror '/ocfs2/ocr2.dbf'

## relocate an existing OCR file ocrconfig -replace ocr '/ocfs1/ocr_new.dbf'

## remove the OCR or OCRMirror fileocrconfig -replace ocrocrconfig -replace ocrmirror

Voting Disk

The voting disk as I mentioned in the architecture is used to resolve membership issues in the event of a partitioned cluster, the voting disk protects data integrity.

querying crsctl query css votedisk

adding crsctl add css votedisk <file>

deleting crsctl delete css votedisk <file>

\

RAC Backups and Recovery

Backups and recovery is very similar to a single instance database. This article covers only the specific issues that surround RAC backups and recovery, I have already written a article on standard Oracle backups and recovery.

Backups can be different depending on the the size of the company

small company - may use tools such as tar, cpio, rsync medium/large company - Veritas Netbackup, RMAN

Enterprise company - SAN mirroring with a backup option like Netbackup or RMAN

Oracle RAC can use all the above backup technologies, but Oracle prefers you to use RMAN oracle own backup solution.

Backup Basics

Oracle backups can be taken hot or cold, a backup will comprise of the following

Datafiles Control Files

Archive redolog files

Parameter files (init.ora or SPFILE)

Databases have now grown to very large sizes well over a terabyte in size in some cases, thus tapes backups are not used in these cases but sophisticated disk mirroring have taken their place. RMAN can be used in either a tape or disk solution, it can even work with third-party solutions such as Veritas Netbackup.

In a Oracle RAC environment it is critical to make sure that all archive redolog files are located on shared storage, this is required when trying to recover the database, as you need access to all archive redologs. RMAN can use parallelism when recovering, the node that performs the recovery must have access to all archived redologs, however, during recovery only one node applies the archived logs as in a standard single instance configuration.

Oracle RAC also supports Oracle Data Guard, thus you can have a primary database configured as a RAC and a standby database also configured as a RAC.

Instance Recovery

In a RAC environment there are two types of recovery

Crash Recovery - means that all instances have failed, thus they all need to be recovered

Instance Recovery - means that one or more instances have failed. this instance can then be recovered by the surviving instances

Redo information generated by an instance is called a thread of redo. All log files for that instance belong to this thread, an online redolog file belongs to a group and the group belongs to a thread. Details about log group file and thread association details are stored in the control

file. RAC databases have multiple threads of redo, each instance has one active thread, the threads are parallel timelines and together form a stream. A stream consists of all the threads of redo information ever recorded, the streams form the timeline of changes performed to the database.

Oracle records the changes made to a database, these are called change vectors. Each vector is a description of a single change, usually a single block. A redo record contains one or more change vectors and is located by its Redo Byte Address (RBA) and points to a specific location in the redolog file (or thread). It will consist of three components

log sequence number block number within the log

byte number within the block

Checkpoints are the same in a RAC environment and a single instance environment, I have already discussed checkpoints, when a checkpoint needs to be triggered, Oracle will look for the thread checkpoint that has the lowest checkpoint SCN, all blocks in memory that contain changes made prior to this SCN across all instances must be written out to disk. I have discussed how to control recovery in my Oracle section and this applies to RAC as well.

Crash Recovery

Crash recovery is basically the same for a single instance and a RAC environment, I have a complete recovery section in my Oracle section, here is a note detailing the difference

For a single instance the following is the recovery process

1. The on-disk block is the starting point for the recovery, Oracle will only consider the block on the disk so the recovery is simple. Crash recovery will automatically happen using the online redo logs that are current or active

2. The starting point is the last full checkpoint. The starting point is provided by the control file and compared against the same information in the data file headers, only the changes need to be applied

3. The block specified in the redolog is read into cache, if the block has the same timestamp as the redo record (SCN match) the redo is applied.

For a RAC instance the following is the recovery process

1. A foreground process in a surviving instance detects an "invalid block lock" condition when a attempt is made to read a block into the buffer cache. This is an indication that an instance has failed (died)

2. The foreground process sends a notification to instance system monitor (SMON) which begin to search for dead instances. SMON maintains a list of all the dead instances and invalid block locks. Once the recovery and cleanup has finished this list is updated.

3. The death of another instance is detected if the current instance is able to acquire that instance's redo thread locks, which is usually held by an open and active instance.

Oracle RAC uses a two-pass recovery, because a data block could have been modified in any of the instances (dead or alive), so it needs to obtain the latest version of the dirty block and it uses PI (Past Image) and Block Written Record (BWR) to archive this in a quick and timely fashion.

Block Written Record (BRW)

The cache aging and incremental checkpoint system would write a number of blocks to disk, when the DBWR completes a data block write operation, it also adds a redo record that states the block has been written (data block address and SCN). DBWn can write block written records (BWRs) in batches, though in a lazy fashion. In RAC a BWR is written when an instance writes a block covered by a global resource or when it is told that its past image (PI) buffer it is holding is no longer necessary.

Past Image (PI)

This is was makes RAC cache fusion work, it eliminates the write/write contention problem that existed in the OPS database. A PI is a copy of a globally dirty block and is maintained in the database buffer cache, it can be created and saved when a dirty block is shipped across to another instance after setting the resource role to global. The GCS is responsible for informing an instance that its PI is no longer needed after another instance writes a newer (current) version of the same block. PI's are discarded when GCS posts all the holding instances that a new and consistent version of that particular block is now on disk.

I go into more details about PI's in my cache fusion section.

The first pass does not perform the actual recovery but merges and reads redo threads to create a hash table of the blocks that need recovery and that are not known to have been written back to the datafiles. The checkpoint SCN is need as a starting point for the recovery, all modified blocks are added to the recovery set (a organized hash table). A block will not be recovered if its BWR version is greater than the latest PI in any of the buffer caches.

The second pass SMON rereads the merged redo stream (by SCN) from all threads needing recovery, the redolog entries are then compared against a recovery set built in the first pass and any matches are applied to the in-memory buffers as in a single pass recovery. The buffer cache is flushed and the checkpoint SCN for each thread is updated upon successful completion.

Cache Fusion Recovery

I have a detailed section on cache fusion, this section covers the recovery, cache fusion is only used in RAC environments, as additional steps are required, such as GRD reconfiguration, internode communication, etc. There are two types of recovery

Crash Recovery - all instances have failed Instance Recovery - one instance has failed

In both cases the threads from failed instances need to be merged, in a instance recovery SMON will perform the recovery where as in a crash recovery a foreground process performs the recovery.

The main features (advantages) of cache fusion recovery are

Recovery cost is proportional to the number of failures, not the total number of nodes It eliminates disk reads of blocks that are present in a surviving instance's cache

It prunes recovery set based on the global resource lock state

The cluster is available after an initial log scan, even before recovery reads are complete

In cache fusion the starting point for recovery of a block is its most current PI version, this could be located on any of the surviving instances and multiple PI blocks of a particular buffer can exist.

Remastering is the term used that describes the operation whereby a node attempting recovery tries to own or master the resource(s) that were once mastered by another instance prior to the failure. When one instance leaves the cluster, the GRD of that instance needs to be redistributed to the surviving nodes. RAC uses an algorithm called lazy remastering to remaster only a minimal number of resources during a reconfiguration. The entire Parallel Cache Management (PCM) lock space remains invalid while the DLM and SMON complete the below steps

1. IDLM master node discards locks that are held by dead instances, the space is reclaimed by this operation is used to remaster locks that are held by the surviving instance for which a dead instance was remastered

2. SMON issues a message saying that it has acquired the necessary buffer locks to perform recovery

Lets look at an example on what happens during a remastering, lets presume the following

Instance A masters resources 1, 3, 5 and 7 Instance B masters resources 2, 4, 6, and 8

Instance C masters resources 9, 10, 11 and 12

Instance B is removed from the cluster, only the resources from instance B are evenly remastered across the surviving nodes (no resources on instances A and C are affected), this reduces the amount of work the RAC has to perform, likewise when a instance joins a cluster only minimum amount of resources are remastered to the new instance.

Before Remastering After Remastering

You can control the remastering process with a number of parameters

_gcs_fast_config enables fast reconfiguration for gcs locks (true|false)

_lm_master_weight controls which instance will hold or (re)master more resources than others

_gcs_resources controls the number of resources an instance will master at a time

you can also force a dynamic remastering (DRM) of an object using oradebug

force dynamic remastering (DRM)

## Obtain the OBJECT_ID form the below table SQL> select * from v$gcspfmaster_info;

## Determine who masters itSQL> oradebug setmypidSQL> oradebug lkdebug -a <OBJECT_ID>

## Now remaster the resource SQL> oradebug setmypidSQL> oradebug lkdebug -m pkey <OBJECT_ID>

The steps of a GRD reconfiguration is as follows

Instance death is detected by the cluster manager Request for PCM locks are frozen

Enqueues are reconfigured and made available

DLM recovery

GCS (PCM lock) is remastered

Pending writes and notifications are processed

I Pass recovery

o The instance recovery (IR) lock is acquired by SMON

o The recovery set is prepared and built, memory space is allocated in the SMON PGA

o SMON acquires locks on buffers that need recovery

II Pass recovery

o II pass recovery is initiated, database is partially available

o Blocks are made available as they are recovered

o The IR lock is released by SMON, recovery is then complete

o The system is available

Graphically it looks like below

RAC Performance

I have already discussed basic Oracle tuning, in this section I will mainly dicuss Oracle RAC tuning. First lets review the best pratices of a Oracle design regarding the application and database

Optimize connection management, ensure that the middle tier and programs that connect to the database are efficent in connection management and do not log on or off repeatedly

Tune the SQL using the available tools such as ADDM and SQL Tuning Advisor

Ensure that applications use bind variables, cursor_sharing was introduced to solve this problem

Use packages and procedures (because they are compiled) in place of anonymous PL/SQL blocks and big SQL statements

Use locally managed tablespaces and automatic segment space management to help performance and simplify database administration

Use automatic undo management and temporary tablespace to simplify administration and increase performance

Ensure you use large caching when using sequences, unless you cannot afford to lose sequence during a crash

Avoid using DDL in production, it increases invalidations of the already parsed SQL statements and they need to be recompiled

Partion tables and indexes to reduce index leaf contention (buffer busy global cr problems)

Optimize contention on data blocks (hot spots) by avoiding small tables with too many rows in a block

Now we can review RAC specific best practices

Consider using application partitioning (see below) Consider restricting DML-intensive users to using one instance, thus reducing cache

contention

Keep read-only tablespaces away from DML-intensive tablespaces, they only require minimum resources thus optimizing Cache Fusion performance

Avoid auditing in RAC, this causes more shared library cache locks

Use full tables scans sparingly, it causes the GCS to service lots of block requests, see table v$sysstat column "table scans (long tables)"

if the application uses lots of logins, increase the value of sys.audsess$ sequence

Partitioning Workload

Workload partitioning is a certian type of workload that is executed on an instance, that is partitioning allows users who access the same set of data to log on to the same instance. This limits the amount of data that is shared between instances thus saving resources used for messaging and Cache Fusion data block transfer.

You should consider the following when deciding to implement partitioning

If the CPU and private interconnects are of high performance then there is no need to to partition

Partitioning does add complexity, thus if you can increase CPU and the interconnect performance the better

Only partition if performance is betting impacted

Test both partitioning and non-partitioning to what difference it makes, then decide if partitioning is worth it

RAC Wait Events

An event is an operation or particular function that the Oracle kernel performs on behalf of a user or a Oracle background process, events have specific names like database event. Whenever a session has to wait for something, the wait time is tracked and charged to the event that was associated with that wait. Events that are associated with all such waits are known as wait events. The are a number of wait classes

Commit Scheduler

Application

Configuration

User I/O

System I/O

Concurrency

Network

Administrative

Cluster

Idle

Other

There are over 800 different events spread across the above list, however you probably will only deal with about 50 or so that can improve performance.

When a session requests access to a data block it sends a request to the lock master for proper authorization, the request does not know if it will receive the block via Cache Fusion or a permission to read from the disk. Two placeholder events

global cache cr request (consistent read - cr) global cache curr request (current - curr)

keep track of the time a session spends in this state. There are number of types of wait events regarding access to a data block

Wait Event Contention

type Description

gc current block 2-way

write/write

an instance requests authorization for a block to be accessed in current mode to modify a block, the instance mastering the resource receives the request. The master has the current version of the block and sends the current copy of the block to the requestor via Cache Fusion and keeps a Past Image (.PI)

If you get this then do the following

Analyze the contention, segments in the "current blocks received" section of Use application partitioning scheme

Make sure the system has enough CPU power

Make sure the interconnect is as fast as possible

Ensure that socket send and receive buffers are configured correctly

gc current block 3-way

write/write

an instance requests authorization for a block to be accessed in current mode to modify a block, the instance mastering the resource receives the request and forwards it to the current holder of the block, asking it to relinquish ownership. The holding instance sends a copy of the current version of the block to the requestor via Cache Fusion and transfers the exclusive lock to the requesting instance. It also keeps a past Image (PI).

Use the above actions to increase the performance

gc current block 2-way

write/read The difference with the one above is that this sends a copy of the block thus keeping the current copy.

gc current write/read The difference with the one above is that this sends a copy of the block thus keeping the current copy.

block 3-way

gc current block busy

write/write

The requestor will eventually get the block via cache fusion but it is delayed due to one of the following

The block was being used by another session on another session was delayed as the holding instance could not write the corresponding redo record immediately

If you get this then do the following

Ensure the log writer is tuned

gc current buffer busy

localThis is the same as above (gc current block busy), the difference is that another session on the same instance also has requested the block (hence local contention)

gc current block congested

none This is caused if heavy congestion on the GCS, thus CPU resources are stretched

Enqueue Tuning

Oracle RAC uses a queuing mechanism to ensure proper use of shared resources, it is called Global Enqueue Services (GES). Enqueue wait is the time spent by a session waiting for a shared resource, here are some examples of enqueues:

updating the control file (CF enqueue) updating an individual row (TX enqueue)

exclusive lock on a table (TM enqueue)

Enqueues can be managed by the instance itself others are used globally, GES is responsible for coordinating the global resources. The formula used to calculate the number of enqueue resources is as below

                     GES Resources = DB_FILES + DML_LOCKS + ENQUEUE_RESOURCES + PROCESS + TRANSACTION x (1 + (N - 1)/N)

                      N = number of RAC instances

displaying enqueues stats

SQL> column current_utilization heading currentSQL> column max_utilization heading max_usageSQL> column initial_allocation heading initialSQL> column resource_limit format a23;

SQL> select * from v$resource_limit;

AWR and RAC

I have already discussed AWR in a single instance environment, so for a quick refresh take a look and come back here to see how you can use it in a RAC environment.

From a RAC point of view there are a number of RAC-specific sections that you need to look at in the AWR, in the report section is a AWR of my home RAC environment, you can view the whole report here.

RAC AWR Section Report Description

Number of Instances instances lists the number of instances from the beginning and end of the AWR report

Instance global cache load profile

global cache

information about the interinstance cache fusion data block and messaging traffic, because my lightweight here is a more heavy used RAC example

Global Cache Load Profile~~~~~~~~~~~~~~~~~~~~~~~~~            Per Second        Per Transaction                                                 ---------------          ---------------Global Cache blocks received:       315.37                   12.82Global Cache blocks served:          240.30                   9.67GCS/GES messages received:          525.16                   20.81GCS/GES messages sent:                765.32                   30.91

The first two statistics indicate the number of blocks transferred to or from this instance, thus if you are using a 8K block size

        Sent:        240 x 8,192 = 1966080 bytes/sec = 2.0 MB/sec        Received:  315 x 8,192 = 2580480 bytes/sec = 2.6 MB/sec

to determine the amount of network traffic generated due to messaging you first need to find the average message size (this was 193 on my system)

    select sum(kjxmsize * (kjxmrcv + kjxmsnt + kjxmqsnt)) / sum((kjxmrcv + kjxmsnt + kjxmqsnt)) "avg Message size" from x$kjxm         where kjxmrcv > 0 or kjxmsnt > 0 or kjxmqsnt > 0;

then calculate the amount of messaging traffic on this network

    193 (765 + 525) = 387000 = 0.4 MB

to calculate the total network traffic generated by cache fusion

     = 2.0 + 2.6 + 0.4 = 5 MBytes/sec     = 5 x 8 = 40 Mbits/sec

The DBWR Fusion writes statistic indicates the number of times the local DBWR was forced to write a block to disk due to remote instances, this number should be low.

Glocal cache efficiency percentage

global cache efficiency

this section shows how the instance is getting all the data blocks it needs. The best order is the following

Local cache Remote cache

Disk

The first two give the cache hit ratio for the instance, you are looking for a value less than 10%, if you are getting

higher values then you may consider application partitioning.

GCS and GES - workload characteristics

GCS and GES workload

this section contains timing statistics for global enqueue and global cache. As a general rule you are looking for

All timings related to CR (Consistent Read) processing block should be less than 10 msec

All timings related to CURRENT block processing should be less than 20 msec

Messaging statistics messaging

The first section relates to sending a message and should be less than 1 second.

The second section details the breakup of direct and indirect messages, direct messages are sent by a instance foreground or the user processes to remote instances, indirect are messages that are not urgent and are pooled and sent.

Service statistics Service stats shows the resources used by all the service instance supports

Service wait class statistics

Service wait class summarizes waits in different categories for each service

Top 5 CR and current block segements

Top 5 CR and current blocks

conatns the names of the top 5 contentious segments (table or index). If a table or index has a very high percentage of CR and Current block transfers you need to investigate. This is pretty much like a normal single instance.

Cluster Interconnect

As I stated above the interconnect it a critical part of the RAC, you must make sure that this is on the best hardware you can buy. You can confirm that the interconnect is being used in Oracle 9i and 10g by using the command oradebug to dump information out to a trace file, in Oracle 10g R2 the cluster interconnect is also contained in the alert.log file, you can view my information from here.

interconnect

SQL> oradebug setmypidSQL> oradebug ipc

Note: look in the user_dump_dest directory, the trace will be there