Capacity Planning and Architecture - Expert Oracle RAC Performance Diagnostics and Tuning - page 34

During these block transfer requests between instances using the interconnect, the GCS process plays a
significant role as the master/keeper of all requests between instances. GCS tracks the location and status of data
blocks as well as the access privileges of various instances. Oracle uses the GCS for cache coherency when the current
version of a data block is on one instance's buffer cache and another instance requests that block for modification.
When multiple instances require access to a block, and a different instance masters the block, the GCS resources
track the movement of blocks through the master instance. Because of block transfer between instances, multiple
copies of the same block could be on different instances. These blocks in different instances have different resource
characteristics. These characteristics are identified by the following factors, which we discuss in the next two sections:

• Resource mode

• Resource role

Resource Mode
Resource mode is determined by various factors such as who is the original holder of the block, what operation is the
block acquired to perform, what operation is the requesting holder intending to perform, what will be the outcome of
the operation, and so forth. Table 2-1 lists the resource modes, its identifier, and a description of the resource mode.

Table 2-1. Resource Modes

Resource Mode Identifier Description

Null N Nodes holding blocks at this level convey no access rights.

Shared S This level indicates that the block is being held in protected read mode. That is,

multiple instances have access to read this block but cannot modify it.

Exclusive X This indicates that the resource is held in Exclusive mode. Whereas consistent

versions of the older blocks are available, other processes or nodes cannot write
to the resource.

Resource Role
Role indicates if the mode is maintained local to the instance or if it's maintained across multiple instances, hence,
at a global level. Table 2-2 illustrates the different roles and their descriptions.

Table 2-2. Resource Roles

Role Description

Local When the block, for the first time, is read into an instance's cache, and no other instance in the cluster

has read the same block or is holding a copy of the block, then the block has a local role.

Global If the block that was originally acquired has been modified by the holding instance, and based on a

request from another instance, has copied the block, the block that was originally on one node is now
present on multiple nodes and therefore has a global role.