System Global Area (SGA)
The System Global Area (SGA) is a vital component of Oracle Database’s memory architecture, serving as a read/write memory area that, together with Oracle’s background processes, forms the foundation of a database instance. Here are key points about the SGA:
- Database Instance Foundation: The SGA, along with the Oracle background processes, constitutes the core of a database instance. All server processes responsible for executing user operations can access and retrieve information from the instance SGA. While the background processes work closely with the SGA, they are distinct entities and do not reside within the SGA.
- Unique to Each Instance: Every database instance possesses its own dedicated SGA. Oracle Database automatically allocates memory for the SGA during the startup of an instance and releases this memory when the instance is shut down.
- Memory Components: The SGA is composed of several memory components, each serving a specific purpose and designed to fulfill distinct memory allocation requirements. These components operate using units of contiguous memory known as granules, and the size of these granules is platform-specific and determined by the overall size of the SGA.
- V$SGASTAT View: To access information about the SGA components and their respective memory usage, you can query the
V$SGASTATview, which provides valuable insights into the SGA’s performance and resource utilization.
The most important SGA components are the following:
- Database Buffer Cache
- In-Memory Area
- Redo Log Buffer
- Shared Pool
- Large Pool
- Java Pool
- Streams Pool
- Fixed SGA
The Database Buffer Cache is a crucial memory area in Oracle Database responsible for storing copies of data blocks read from data files. It plays a vital role in optimizing database performance and reducing physical I/O operations. Here’s an overview and summary of the key points about the Database Buffer Cache:
Database Buffer Cache:
The Database Buffer Cache, also known as the buffer cache, is a region in memory that temporarily holds currently or recently used data blocks. Multiple users connected to a database instance share access to this cache.
- Purpose: The Database Buffer Cache, often referred to as the buffer cache, serves as a crucial memory area in Oracle Database. It is dedicated to temporarily holding data blocks read from data files.
- Optimizing I/O: The primary objective of the buffer cache is to optimize physical I/O operations. It achieves this by allowing the database to update data blocks in memory and store information about changes before committing them to disk, thereby reducing the need for immediate disk writes.
- Buffer States: Buffers within the buffer cache can exist in three states: Unused (available for use), Clean (containing read-consistent data), or Dirty (holding modified data yet to be written to disk).
- Buffer Modes: Oracle Database uses two buffer modes for data retrieval: Current Mode (for accessing data as it currently exists, including uncommitted changes) and Consistent Mode (retrieving a read-consistent version of a data block).
- Buffer I/O: Buffer I/O refers to the process of reading and writing buffers within the cache. If a requested buffer is not in memory, the database performs physical I/O to copy it from disk into memory and then performs logical I/O to read the cached buffer.
- Buffer Replacement Algorithms: The buffer cache employs algorithms for efficient buffer replacement. The primary method is the LRU-based, block-level replacement algorithm, but Oracle Database 12c introduced temperature-based, object-level replacement for big table caching.
- Buffer Writes: The Database Writer (DBW) process periodically writes cold, dirty buffers to disk. This process occurs in various situations, such as when clean buffers are needed or a threshold is reached.
- Buffer Reads: When unused buffer space is low, the database removes buffers from the cache. Flash cache can be used to accelerate cache reads by storing buffer bodies.
- Buffer Pools: The buffer cache can be divided into separate buffer pools, such as the default, keep, and recycle pools, each tailored for different block management strategies.
- Impact of Full Table Scans: Full table scans can potentially impact the buffer cache by reading large amounts of data into memory. Oracle Database uses algorithms to balance cache usage and scan efficiency.
- CACHE Attribute: The CACHE attribute can be used to modify caching behavior for specific tables, allowing blocks to age out of the cache rather than forcing them to be pinned.
- KEEP Attribute: The KEEP attribute can be used to load blocks from large tables into the keep buffer pool, ensuring that frequently accessed objects remain in memory.
- Force Full Database Caching Mode: Oracle Database offers a “Force Full Database Caching Mode,” introduced in Oracle Database 12c Release 1, which considers the entire database for caching in the buffer cache, including NOCACHE LOBs, enhancing overall caching behavior.