Oracle Tips by Burleson

RAC Database Instance Tips

RAC Database has multiple database instances to access and manage a single database system. With the shared disk architecture, Database consists of a single set of physical data files for data and they can be accessed by multiple database instances. As shown in the Fig. 4.2, each of the instances resides on a separate host and forms its own set of background processes and memory buffers. Thus, RAC enables access to a single database via multiple database instances.

When the database is not a RAC system, it has one instance and one database. Sometimes, instance and database are construed to be the same. In that case, it is called stand-alone database system.

As an example,

Database Name        : NYDB50

Instance-1 Name      : NYDB51
Instance-2 Name      : NYDB52               
Instance-3 Name      : NYDB53

The parameter DB_NAME will have the value of NYDB50 and this represents the name of the database. Moreover, the parameter instance_name will be one of the above names. All these instances provide access to the same database, which is name NYDB50.

Figure 4.2 Multi-Instance RAC Database System – At a Glance

Let's start our examination of the Oracle Architecture by looking at the memory pools and the background processes in a single-instance database. Then we will extend the study to look at the extra processes and structures that are formed in case of a RAC configuration.

Database Instance

A typical RAC database instance is very much like a stand-alone database system. It has all frills and bells of a typical database instance. However, it has some extra processes, memory structures and logical structures. Since the RAC database system has to maintain concurrency of data across multiple instances, it creates additional structures to manage and coordinate the resources.

The Oracle Instance has various components to support the database processing. The memory components are broadly categorized as System Global Area (SGA) and Program Global Area (PGA).

System Global Area (SGA)

The system global area (SGA) consists of various memory components. A component represents a pool of memory used to satisfy a particular class of memory allocation requests. The most commonly configured memory components include the database buffer cache, shared pool, java pool, large pool, streams pool, data dictionary cache and redo log buffer. PGA consists of session specific information that contains data and control structures.

RAC System Global Area

The size of the SGA is determined by several initialization parameters. The following shows the parameters that influence the SGA size. However, when the initialization parameter ‘sga_target’ is set to greater than zero, the automatic SGA configuration kicks in. We will cover more details in a later part of this chapter.

Database Block Buffers

The database buffer cache holds copies of the data blocks read from the data files. The term ‘data block’ is used to describe a block containing table data, index data, clustered data, and so on. Basically, it is a block that contains data. All user processes concurrently connected to the instance share access to the database buffer cache. The database buffer cache is logically segmented into multiple sets, which reduces contention on multiprocessor systems.

This area of the SGA contains only the buffers themselves and not their control structures. For each buffer, there is corresponding buffer header in the variable area of the SGA.

From Oracle8 release onwards, the buffer cache contains three buffer pools for different type of data usage. They are DEFAULT, KEEP, and RECYCLE. These three buffer pools have separate allocations of buffers and LRU lists that manage buffers.

• RECYCLE buffer pool is used to store blocks that are virtually never used after the initial read. This pool eliminates the data blocks from the memory in next to no time when longer needed. This is more like a work area for the blocks.

• The KEEP pool is for the allocation of buffers for the objects that are accessed with medium frequency or those for which a consistent response time is desirable. This buffer pool retains the schema object’s data blocks in memory.
   

• The DEFAULT buffer pool contains data blocks from schema objects that are not assigned to any buffer pool and as well as for the schema objects that are explicitly assigned to the DEFAULT pool.

The database block buffers act as the holding area for data used by the user and DBWR processes. Any data that gets to the user from the database files, or data that goes into the database files from the user or other processes, passes through the database block buffers (unless direct insert or direct read is used for data loading, sorting, or hashing operations).

The database block buffers in releases prior to Oracle9i had to be of uniform size, 2, 4, 8, 16, or for 64 bit OS, the 32 kilobytes in size. From Oracle9i, the database has a default database cache block size, but other sizes (2K, 4K, 8K, 16K, or 32K) can also be specified. Based on the tablespace size, appropriate Cache is employed to retrieve and manage the buffers in the SGA.

In the RAC database system, the database block buffers from each of the participating instance, through the process of ‘cache fusion’, are merged to form a logical database block buffer area that becomes many times larger than could be supported in a single instance.

Cache Fusion and Inter-Instance database buffer transfers are fully covered in Chapter-9, Cache Fusion and Inter-Instance Coordination.

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