RAC for Beginners

Real Application Clusters (RAC)


Oracle RAC, introduced with Oracle9i, is the successor to Oracle Parallel Server (OPS). Oracle RAC allows multiple instances to access the same database (storage) simultaneously. RAC provides fault tolerance, load balancing, and performance benefits by allowing the system to scale out, and at the same time since all nodes access the same database, the failure of one instance will not cause the loss of access to the database.

Oracle RAC 10g is a shared disk subsystem. All nodes in the cluster must be able to access all of the data, redo log files, control files and parameter files for all nodes in the cluster. The data disks must be globally available in order to allow all nodes to access the database. Each node has its own redo log file(s) and UNDO tablespace, but the other nodes must be able to access them (and the shared control file) in order to recover that node in the event of a system failure.

The difference between Oracle RAC and OPS is the addition of Cache Fusion. With OPS a request for data from one node to another required the data to be written to disk first, then the requesting node can read that data. With cache fusion, data is passed along a high-speed interconnect using a sophisticated locking algorithm.

With Oracle RAC 10g, the data files, redo log files, control files, and archived log files reside on shared storage on raw-disk devices, a NAS, ASM, or on a clustered file system

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) this 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


LMON

The background Global Enqueue Service Monitor (LMON) monitors the entire cluster to manage global resources. LMON manages instance and process failures and the associated recovery for the Global Cache Service (GCS) and Global Enqueue Service (GES). In particular, LMON handles the part of recovery associated with global resources. LMON-provided services are also known as cluster group services (CGS)

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


LCKx

The LCK process manages instance global enqueue requests and cross-instance call operations. Workload is automatically shared and balanced when there are multiple Global Cache Service Processes (LMSx).

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

LMSx

The Global Cache Service Processes (LMSx) are the processes that handle remote Global Cache Service (GCS) messages. Current Real Application Clusters software provides for up to 10 Global Cache Service Processes. The number of LMSx varies depending on the amount of messaging traffic among nodes in the cluster. The LMSx handles the acquisition interrupt and blocking interrupt requests from the remote instances for Global Cache Service resources. For cross-instance consistent read requests, the LMSx will create a consistent read version of the block and send it to the requesting instance. The LMSx also controls the flow of messages to remote instances.

This process is called as Global Cache service process.This process maintains statuses of datafiles and each cahed block by recording information in a Global Resource Dectory(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.


LMDx

The Global Enqueue Service Daemon (LMD) is the resource agent process that manages Global Enqueue Service (GES) resource requests. The LMD process also handles deadlock detection Global Enqueue Service (GES) requests. Remote resource requests are requests originating from another instance.

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

DIAG

The diagnose daemon is a Real Application Clusters background process that captures diagnostic data on instance process failures. No user control is required for this demo.

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.

GTX0-j

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

RMSn

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

RSMN

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

CRS

CRS (Cluster Ready Services) is a new feature for 10g Real Application Clusters that provides a standard cluster interface on all platforms and performs new high availability operations not available in previous versions. CRS manages cluster database functions including node membership, group services, global resource management, and high availability. CRS serves as the clusterware software for all platforms. It can be the only clusterware or run on top of vendor clusterware such as Sun Cluster, HP Serviceguard, etc.


CRS automatically starts the following resources:
· Nodeapps
o Virtual Internet Protocol(VIP) address for each node
o Global Services Daemon
o Oracle Net Listeners
o Oracle Network Services (ONS)
· Database Instance
· Services


Oracle Clusterware (Cluster Ready Services in 10g/ Cluster Manager in 9i) - provides infrastructure that binds multiple nodes that then operate as single server. Clusterware monitors all components like instances and listeners. There are two important components in Oracle clusterware, Voting Disk and OCR (Oracle Cluster Registry)

OCR & Voting Disk

Oracle, 10g RAC, provided its own cluster-ware stack called CRS. The main file components of CRS are the Oracle Cluster Repository (OCR) and the Voting Disk.
The OCR contains cluster and database configuration information for RAC and Cluster Ready Services (CRS). Some of this information includes the cluster node list, cluster database instance-to-node mapping information, and the CRS application resource profiles. The OCR contains configuration details for the cluster database and for high availability resources such as services, Virtual Inerconnect Protocoal (VIP) addresses.

The Voting Disk is used by the Oracle cluster manager in various layers. The Node Monitor (NM) uses the Voting Disk for the Disk Hearbeat, which is essential in the detection and resolution of cluster "split brain".


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 insatnce 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 cahce fusion

Cache Fusion and Global Cache Service (GCS)
Memory-to-memory copies between buffer caches over high-speed interconnects

· fast remote access times
· memory transfers for write or read access
· transfers for all types (e.g data, index, undo, headers )
· Cache coherency across the cluster
· globally managed access permissions to cached data
· GCS always knows whether and where a data block is cached
· a local cache miss may result in remote cache hit or disk read

@@@ Article is still under edit... will be adding more info @@@@