11 Database Memory Allocation

11.1 About Database Memory Caches and Other Memory Structures

Oracle Database stores information in memory caches and on disk. Memory access is much faster than disk access. Disk access (physical I/O) take a significant amount of time, compared to memory access, typically in the order of 10 milliseconds. Physical I/O also increases the CPU resources required due to the path length in device drivers and operating system event schedulers. For this reason, it is more efficient for data requests of frequently accessed objects to be perform by memory, rather than also requiring disk access. Proper sizing and effective use of Oracle Database memory caches greatly improves database performance.

The main Oracle Database memory caches that affect performance include:

• Database buffer cache

  The database buffer cache stores data blocks read from disk.

• Redo log buffer

  The redo log buffer stores redo entries of changes made to data blocks in the buffer cache.

• Shared pool

  The shared pool caches many different types of data and is mainly comprised of the following components:

  • Library cache

  • Data dictionary cache

  • Server result cache

• Large pool

  The large pool provides large memory allocations for the following Oracle Database features:

  • Shared server architecture

  • Parallel query

  • Recovery Manager (RMAN)

• Java pool

  The Java pool stores session-specific Java code and Java Virtual Machine (JVM) data.

• Streams pool

  The Streams pool provides memory for Oracle Streams processes.

• Process-private memory

  Process-private memory includes memory used for operations such as sorting and hash joins.

• In-Memory Column Store (IM column store)

  Starting in Oracle Database 12c Release 1 (12.1.0.2), the IM column store is an optional, static SGA pool that stores copies of tables and partitions. In the IM column store, data is stored in a special columnar format, which improves performance of operations such as scans, joins, and aggregations.

  Note:

  The IM column store does not replace the buffer cache, but acts as a supplement so that both memory areas can store the same data in different formats.

11.2 Database Memory Management Methods

The goal of memory management is to reduce the physical I/O overhead as much as possible, either by making it more likely that the required data is in memory, or by making the process of retrieving the required data more efficient. To achieve this goal, proper sizing and effective use of Oracle Database memory caches is essential.

Oracle Database provides the following methods to manage database memory:

• Automatic Memory Management

• Automatic Shared Memory Management

• Manual Shared Memory Management

• Automatic PGA Memory Management

• Manual PGA Memory Management

11.3 Using Automatic Memory Management

To use automatic memory management, set the following initialization parameters:

• MEMORY_TARGET

  The MEMORY_TARGET initialization parameter specifies the target memory size. The database tunes to the value specified for this parameter, redistributing memory as needed between the SGA and the instance PGA. This parameter is dynamic, so its value can be changed at any time without restarting the database.

• MEMORY_MAX_TARGET

  The MEMORY_MAX_TARGET initialization parameter specifies the maximum memory size. The value specified for this parameter serves as the limit to which the MEMORY_TARGET initialization parameter can be set. This parameter is static, so its value cannot be changed after instance startup.

If you need tuning advice for the MEMORY_TARGET parameter, then use the V$MEMORY_TARGET_ADVICE view.

11.4 Monitoring Memory Management

Table 11-1 lists the views that provide information about memory resize operations.