FAQ-RAC-Basic

RAC Basic

1. What is RAC? RAC is a clustering solution that provides high availability of databases on instance level by providing instance failover feartue.

2. What is Major components of RAC?

Shared Storage

Public IP

private IP

Virtual IP

3. How is RAC different from standby or DR? RAC is the solution for instance, software and hardware level failures but Data guard is the recommended solution for the database or server failures.

4. What is Memory architecture in RAC? As we know , RAC builds with two or more instances. When a data block is already read by an RAC instance and another instance also needs the same block. In this situation , it is easy to get a data block from the instance which has the block in SGA rather than reading from the disk. To enable inter instance communication Oracle RAC makes use of interconnects.

5. Explane Cache Fusion?

* Cache Fusionis disk less cache coherence mechanism that provides copies of data blocks directly from one instance’s memory cache to another instance. All block transfers happen through the private interconnects between the rac nodes. * In a Single Node database, an instance looking for data block first checks in Local cache, if block is not in local cache then goes to disk to pull block from disk to cache and return block to client. * In RAC Database there is remote cache so instances should look not only in local cache but on remote cache. If cache is available in local cache then it should return data block from local cache; if data block is not in local cache, instead of going to disk it should first go to remote cache (remote instance) to check if block is available in local cache (via interconnect) This is because accessing a data block from a remote cache is faster than accessing it from disk.

6. Which Components must reside in Shared Storage?

All datafiles

control files

SPFIles

redo log files

OCR and Voting DISK

Services

1. How many services are in RAC? Can you explain it?

Cache fusion Model is dependent on three services

* Global Resource Directory (GRD)

* Global Cache Service (GCS)

* Global En-queue Service (GES)

2. What is GRD?

GRD (Global Resource Directory) resides in the SGA of every node.

* It contains information about the current status of all shared resources like blocks and locks.

* It’s maintained by GES (Global Enqueue Services) and GCS (Global Cache Services).

3. What is GCS?

* GCS is the main controlling process that implements Cache Fusion.

* GCS tracks the location and the status (mode and role) of the data blocks.

* GCS also tracks the access privileges of various instances.

* It is also responsible for block transfers between the instances.

* Upon a request from an instance, GCS organises the block shipping and appropriate lock mode conversions.

* The Global Cache Service is implemented by various background processes such as the Global Cache Service Processes (LMSn) and Global Enqueue Service Daemon (LMD).

4. What is GES?

* The Global Enqueue Service (GES) manages or tracks the status of all the Oracle enqueuing mechanisms. This involves all non Cache fusion intra-instance operations.

* GES performs concurrency control on dictionary cache locks, library cache locks, and the transactions. GES does this operation for resources that are accessed by more than one instance.

5. What is Cluster Ready Services (CRS):

CRS manages the availability of the RAC database by monitoring the status of the Oracle database instances and listeners running on each node in the cluster.

It also manages the startup and shutdown of these instances and listeners, and can initiate automatic failover of instances in the event of a node failure.

6. Cluster Synchronisation Service (CSS)

It's a clusterware component responsible for managing cluster membership and inter-node communication, helping ensure high availability and coordination between nodes in the cluster.

It also monitors the health of each node in the cluster and handles node failures by reconfiguring the cluster and redistributing workload as needed.