Oracle教程 2023-02-26

Oracle 参数 SGA_TARGET 官方解释，作用，如何配置最优化建议

本站中文解释

SGA_TARGET 参数指定了数据库应该使用的System Global Area（SGA）的目标大小。SGA有三种组成内存结构：shared pool，database buffer cache和large pool。该参数控制SGA组件的大小，它可以让Oracle自动决定最优的每个组件的大小。

正确设置SGA_TARGET参数需要考虑以下因素：机器可用内存，应用程序所需要的内存大小，数据库的工作负荷以及服务器的操作系统。最佳的设置取决于以上几个因素，对其中任何一个的影响应该考虑到。

比如，如果服务器可用的内存是1GB，则可以选择将SGA_TARGET设置为750MB。又比如，在Oracle 10G中，建议SGA_TARGET设置不要超过4GB，如果大于4GB，则尽量将其减少到4GB以内。

官方英文解释

SGA_TARGET specifies the total size of all SGA components.

Property	Description
Parameter type	Big integer
Syntax	`SGA_TARGET =` `integer` `[K \| M \| G]`
Default value	`0` (SGA autotuning is disabled for `DEFERRED` mode autotuning requests, but allowed for `IMMEDIATE` mode autotuning requests)
Modifiable	`ALTER SYSTEM`
Modifiable in a PDB	Yes
Range of values	64 MB to operating system-dependent
Basic	Yes

If SGA_TARGET is specified, then the following memory pools are automatically sized:

Buffer cache (DB_CACHE_SIZE)
Shared pool (SHARED_POOL_SIZE)
Large pool (LARGE_POOL_SIZE)
Java pool (JAVA_POOL_SIZE)
Streams pool (STREAMS_POOL_SIZE)
Data transfer cache (DATA_TRANSFER_CACHE_SIZE)

If these automatically tuned memory pools are set to nonzero values, then those values are used as minimum levels by Automatic Shared Memory Management. You would set minimum values if an application component needs a minimum amount of memory to function properly.

The following pools are manually sized components and are not affected by Automatic Shared Memory Management:

Log buffer
Other buffer caches, such as KEEP, RECYCLE, and other block sizes
Fixed SGA and other internal allocations

The memory allocated to these pools is deducted from the total available for SGA_TARGET when Automatic Shared Memory Management computes the values of the automatically tuned memory pools.

In the Default value field, IMMEDIATE mode autotuning requests are necessary to avoid ORA-04031 errors. The DEFERRED and IMMEDIATE modes are reflected in the OPER_MODE column of the V$MEMORY_RESIZE_OPS view.

If Automatic Memory Management is enabled (MEMORY_TARGET is set to a positive value) and SGA_TARGET is also set to a positive value, the SGA_TARGET value acts as the minimum value for the size of the SGA.

Note:

This parameter is optional for pluggable databases (PDBs). When this parameter is set for a PDB, it specifies the maximum SGA that the PDB can use at any time. When this parameter is not set at the PDB level, the PDB has no limit for the amount of SGA it can use, other than the CDB’s SGA size.

To be able to use Resource Manager in a CDB to control the amount of memory each PDB can use:

The NONCDB_COMPATIBLE initialization parameter must be set to FALSE at the CDB level (in the root of the CDB).
The MEMORY_TARGET initialization parameter must not be set at the CDB level.
You must set the SGA_TARGET initialization parameter at the CDB level.
You must set SGA_TARGET in a PDB.

If you set a PDB’s SGA_TARGET value and SGA_TARGET is not set at the CDB level, you will not receive an error message and the PDB’s SGA_TARGET value will not be enforced.
When setting SGA_TARGET in a PDB, the following requirements must be met:
- The value of SGA_TARGET in the PDB must be less than or equal to the SGA_TARGET value at the CDB level.
- The value of SGA_TARGET in the PDB must be less than or equal to the SGA_MAX_SIZE value at the CDB level.
- The value of SGA_TARGET in the PDB must be twice the PDB’s DB_CACHE_SIZE value, if the PDB’s DB_CACHE_SIZE value is set.
- The value of SGA_TARGET in the PDB must be twice the PDB’s SHARED_POOL_SIZE value, if the PDB’s SHARED_POOL_SIZE value is set.
- The value of SGA_TARGET in the PDB must be twice the PDB’s SGA_MIN_SIZE value, if the PDB’s SGA_MIN_SIZE value is set.
When you set SGA_TARGET in a PDB to a value that does not meet these requirements, you receive an error.

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