SGA (System Global Area):
- Database buffer cache
- Redo log buffer
- Shared pool
- Java pool
- Large pool (optional)
- Streams pool
What is SGA in Oracle Database?
The SGA is a group of shared memory structures, known as SGA components, that contain data and control information for one Oracle Database instance. All server and background processes share the SGA. Examples of data stored in the SGA include cached data blocks and shared SQL areas.
What are the main components of SGA?
The main components of the SGA include the database buffer cache, shared pool, and redo log buffers. The database buffer cache contains actual copies of data blocks from the database. When you update a table, the information is first modified in the data buffer and is later written to disk.
What is SGA (System general area allocation)?
The SGA comprises a number of memory components, which are pools of memory used to satisfy a particular class of memory allocation requests.
What are the data structures in the system Global Area (SGA)?
The SGA contains the following data structures: Part of the SGA contains general information about the state of the database and the instance, which the background processes need to access; this is called the fixed SGA. No user data is stored here. The SGA also includes information communicated between processes, such as locking information.
What are the components of PGA?
The two components of PGA When you execute an SQL statement, the source and the execution plan are stored in shared memory because all processes can use that information. Information that is process specific, such as bind variable information and PL/SQL variable values, is contained in the PGA.
What is Oracle SGA parameter?
Parameter SGA_MAX_SIZE holds the maximum size that System Global Area (SGA) can reach for a particular instance. Oracle Database can change its SGA configuration while the instance is running. This allows sizes of the buffer cache, shared pool, and the large pool to be changed without an instance shutdown.
What are the three 3 major components of Oracle database?
An Oracle instance consists of three main parts: System Global Area (SGA), Program Global Area (PGA), and background processes.
Is PGA part of SGA?
Because the PGA is process-specific, it is never allocated in the SGA.
What is Oracle SGA memory?
The System Global Area (SGA) is a group of shared memory structures, known as SGA components, that contain data and control information for one Oracle Database instance. The SGA is shared by all server and background processes. Examples of data stored in the SGA include cached data blocks and shared SQL areas.
What is Oracle SGA size?
The granule size depends on the database version and sometimes on the operating system. In Oracle 9i and earlier, it is 4 MB if the SGA size is less than 128 MB, and 16 MB otherwise. For later releases, it is typically 4 MB if the SGA size is less than 1 GB, and 16 MB otherwise.
What are Oracle components?
The components that make up the core of Oracle Application Server are the Oracle HTTP Server, Oracle Application Server Containers for J2EE, and OracleAS Web Cache.Oracle HTTP Server. ... Oracle Application Server Containers for J2EE. ... OracleAS Web Cache.
What is SGA Oracle architecture?
A system global area (SGA) is a group of shared memory structures that contain data and control information for one Oracle database instance. If multiple users are concurrently connected to the same instance, then the data in the instance's SGA is shared among the users.
What are the main components of a Oracle Database?
The following sections explain the physical database structures of an Oracle database, including datafiles, redo log files, and control files.Datafiles. Every Oracle database has one or more physical datafiles. ... Control Files. ... Redo Log Files. ... Archive Log Files. ... Parameter Files. ... Alert and Trace Log Files. ... Backup Files.
What is Smon and Pmon in Oracle?
PMON and SMON are two required background processes. PMON is the Process Monitor which is responsible for recovering processes when the user process fails. PMON does the process cleanup. SMON is the System Monitor which is responsible for recovering the system after a failure.
What is Pfile and Spfile?
SPFILE is a binary file that contains the same information as the old PFILE. SPFILE permits dynamic changes without requiring you to restart that instance. By default, if you do not specify PFILE in your STARTUP command, Oracle will use server parameter file (SPFILE).
What is Pga_aggregate_limit and Pga_aggregate_target?
By default, PGA_AGGREGATE_LIMIT is set to the greater of 2 GB, 200% of PGA_AGGREGATE_TARGET , and 3 MB times the PROCESSES parameter. It will be set below 200% of PGA_AGGREGATE_TARGET if it is larger than 90% of the physical memory size minus the total SGA size, but not below 100% of PGA_AGGREGATE_TARGET .
What is SGA and PGA size in Oracle?
System global area (SGA)—A shared memory area that contains data buffers and control information for the instance. The SGA is divided into separate buffer areas and data pools. These are described in "SGA Components". Program global area (PGA)—A memory area used by a single Oracle server process.
What is Sga_max_size and Sga_target?
The Oracle sga_max_size parameter sets the hard limit up to which sga_target can dynamically adjust sizes. Usually, sga_max_size and sga_target will be the same value, but there may be times when you want to have the capability to adjust for peak loads.
Which initialization parameter is used to automatically size the SGA?
DB_nK_CACHE_SIZE initialization parameterFixed SGA and other internal allocations are sized using the DB_nK_CACHE_SIZE initialization parameter.
How do you measure SGA size?
Check SGA total size and free space usage in OracleCheck the Usage of SGA. select round(used.bytes /1024/1024 ,2) used_mb. ... Find the Total Size of SGA. SELECT sum(value)/1024/1024 "TOTAL SGA (MB)" FROM v$sga; ... Check size of different pool in SGA.
Automatic Shared Memory Management
All SGA components allocote and deallocate areas in units of granules.
Automatic Memory Management
If you set MEMORY_TARGET, the SGA and PGA are automatically set by oracle. This process is called Automatic Memory Management.
Manual Memory Management
If you manually set all components, this process is called Manual Memory Management.
Database Buffer Cache
It is a part of the SGA that stores the data blocks read from the data files.
Redo Log Buffer
This is the memory area which the queries are stored in the memory before being written to the redo log.
Shared Pool
Shared Pool is the most complex part of SGA. It consists of many sub-structures. I will explain some of these.
SGA Components
In general, the SGA consists of the following sub-components, as can be verified by querying the V$SGAINFO :
SGA Size
Here are two methods that can be used to determine the current SGA's size. All values are in bytes:
Dynamic sizing
When automatic shared memory management is enabled, Oracle will adjust the memory parameters on the fly. To see currently allocated sizes:
How to prevent SGA from being paged out?
To prevent the SGA from being paged out, consider locking the SGA into physical memory by enabling the LOCK_SGA parameter. The database does not use the MEMORY_TARGET and MEMORY_MAX_TARGET parameters when the LOCK_SGA parameter is enabled.
What is the SGA in memory?
The In-Memory Column Store is an optional area of the SGA that stores copies of tables, partitions, and other database objects in a columnar format that is optimized for rapid scans.
What is the granule size of a SGA?
If the SGA size is less than 1 GB, then the granule size is 4MB. If the SGA size is greater than 1 GB, the granule size changes to 16MB. The granule size is calculated and fixed when the database instance starts up. The size does not change during the lifetime of the instance.
Why does paging occur?
Paging occurs when an operating system transfers memory-resident pages to disk solely to load new pages into memory. Many operating systems page to accommodate large amounts of information that do not fit into real memory. On most operating systems, paging reduces performance.
How to determine if a host system is paging?
To determine whether significant paging is occurring on the host system, use operating system utilities to examine the operating system . If significant paging is occurring, then the total system memory may not be large enough to hold the memory caches for which memory is allocated.
When configuring memory, size the memory caches appropriately based on the application's needs?
Conversely, tuning the application's use of the memory caches can greatly reduce resource requirements. Efficient use of the memory caches also reduces the load on related resources, such as latches, CPU, and the I/O system.
What is the in-memory column store?
The In-Memory Column Store stores copies of tables, partitions, and other database objects in the SGA. The In-Memory Column Store does not replace the database buffer cache. Instead, they complement each other so that both memory areas can store the same data in different formats. Rows stored in the In-Memory Column Store are divided into large memory regions in a columnar format. Within each region, a column resides separately in a contiguous area of memory.
What does a full table scan do?
When the user process is performing a full table scan, it reads the blocks of the table into buffers and puts them on the LRU end (instead of the MRU end) of the LRU list. This is because a fully scanned table usually is needed only briefly, so the blocks should be moved out quickly to leave more frequently used blocks in the cache.
What is the buffer cache in SGA?
The database buffer cache is the portion of the SGA that holds copies of data blocks read from datafiles. All user processes concurrently connected to the instance share access to the database buffer cache.
What is SGA in Java?
The SGA comprises a number of memory components, which are pools of memory used to satisfy a particular class of memory allocation requests. Examples of memory components include the shared pool (used to allocate memory for SQL and PL/SQL execution), the java pool (used for java objects and other java execution memory), and the buffer cache (used for caching disk blocks). All SGA components allocate and deallocate space in units of granules. Oracle Database tracks SGA memory use in internal numbers of granules for each SGA component.
How does Oracle process PL/SQL?
Oracle processes PL/SQL program units (procedures, functions, packages, anonymous blocks, and database triggers) much the same way it processes individual SQL statements. Oracle allocates a shared area to hold the parsed, compiled form of a program unit. Oracle allocates a private area to hold values specific to the session that runs the program unit, including local, global, and package variables (also known as package instantiation) and buffers for executing SQL. If more than one user runs the same program unit, then a single, shared area is used by all users, while each user maintains a separate copy of his or her private SQL area, holding values specific to his or her session.
What is SGA in Oracle?
A system global area (SGA) is a group of shared memory structures that contain data and control information for one Oracle database instance. If multiple users are concurrently connected to the same instance, then the data in the instance's SGA is shared among the users.
How does Oracle save memory?
Oracle saves memory by using one shared SQL area for SQL statements run multiple times, which often happens when many users run the same application. Oracle allocates memory from the shared pool when a new SQL statement is parsed, to store in the shared SQL area.
What is Oracle memory?
Oracle uses memory to store information such as the following: Program code. Information about a connected session, even if it is not currently active. Information needed during program execution (for example, the current state of a query from which rows are being fetched)
