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Global Cache Services: Main Function

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

The main function of the global cache service (GCS) is to track the status and location of the data blocks. Status is the resource role and the resource mode. The GCS is the main mechanism by which cache coherency among the multiple caches is maintained.

GCS maintains the modes for blocks in the global role and is responsible for block transfers between the instances. The LMS processes handle the GCS messages and carry out the bulk of the GCS processing. GCS resource coordination is explained in detail in later sections.

Global Enqueue Service (GES)

The global enqueue service (GES) tracks the status of all Oracle enqueuing mechanisms. This involves all non-cache fusion intra-instance operations. The GES performs concurrency control on dictionary cache locks, library cache locks, and transactions. It performs this operation for resources that are accessed by more than one instance.


What exactly is an enqueue? Enqueues are shared memory structures that serialize access to the database resources. For example, when a user updates a row and gets a row-level lock, a TX enqueue is created for that user. If another user subsequently attempts to update that same row, that user's session will block (wait on) the enqueue that the initial user created. They are sometimes referred to as positive waits.

Enqueues are local to one instance, if real application clusters are not enabled. When the real application cluster is enabled, the enqueues can be global to the database. Enqueues are comprised of transaction locks, DML locks, SCN locks, etc.

Enqueues are associated with a session or a transaction. They are held longer than the latches, have finer granularity, more modes than the latches, and protect more database resources. For example, when a table lock is requested, the request is assigned to an enqueue. Oracle can use enqueues in any of three modes: null (N) mode, shared (S) mode, or exclusive (X) mode.

The GES controls access to data files and control files but not for the data blocks. GES processing includes the coordination for enqueues other than the data blocks.

The resources managed by the GES include the following:

Transaction locks ? It is acquired in the exclusive mode when a transaction initiates its first row level change. The lock is held until the transaction is committed or rolled back.

Library Cache locks - When a database object (such as a table, view, procedure, function, package, package body, trigger, index, cluster, or synonym) is referenced during parsing or compiling of a SQL, DML or DDL, PL/SQL, or Java statement, the process parsing or compiling the statement acquires the library cache lock in the correct mode.

Dictionary Cache Locks - Global enqueues are used in the cluster database mode. The data dictionary structure is the same for all Oracle instances in a cluster database, as it is for instances in a single-instance database. However, in real application clusters, Oracle synchronizes all the dictionary caches throughout the cluster. Real application clusters use latches to do this, just as in the case of a single-instance Oracle database.

Table locks ? These are the GES locks that protect the entire table(s). A transaction acquires a table lock when a table is modified. A table lock can be held in any of several modes: null (N), row share (RS), row exclusive (RX), share lock (S), share row exclusive (SRX), or exclusive (X).


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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