Oracle教程 2023-02-26

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

本站中文解释

DB_CACHE_SIZE参数用于为SGA内存池中的数据库缓冲池指定大小。当Oracle数据库从磁盘加载数据页(如用户使用后从磁盘加载查询结果集数据页)时，它将首先在缓冲池中查找该数据页，如果不存在，则从磁盘加载缓冲区。因此，设置数据库缓冲区的大小可以改善系统性能，减少I/O操作和改善数据库系统的总体效率。

DB_CACHE_SIZE参数的正确设置取决于实际的使用情况，而不是某种固定的大小。如果你的系统频繁读取同一个表或数据页，可以初步考虑设置一个较大的db_cache_size。通常情况下，db_cache_size最好设置为内存池最大大小的一半。

官方英文解释

DB_CACHE_SIZE specifies the size of the DEFAULT buffer pool for buffers with the primary block size (the block size defined by the DB_BLOCK_SIZE initialization parameter).

Property	Description
Parameter type	Big integer
Syntax	`DB_CACHE_SIZE =` `integer` `[K \| M \| G]`
Default value	If `SGA_TARGET` is set: If the parameter is not specified, then the default is `0` (internally determined by the Oracle Database). If the parameter is specified, then the user-specified value indicates a minimum value for the memory pool. If `SGA_TARGET` is not set, then the default is either 48 MB or 4 MB * number of CPUs, whichever is greater
Modifiable	`ALTER SYSTEM`
Modifiable in a PDB	Yes
Basic	No

The value must be at least 4M * number of cpus (smaller values are automatically rounded up to this value). A user-specified value larger than this is rounded up to the nearest granule size. A value of zero is illegal because it is needed for the DEFAULT memory pool of the primary block size, which is the block size for the SYSTEM tablespace.

Note:

This parameter is optional for pluggable databases (PDBs). When this parameter is set for a PDB, it indicates a possible minimum value for the PDB usage of the memory pool.

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.
If the SGA_TARGET initialization parameter is set at the CDB level, then the following requirement must be met:
- The value of DB_CACHE_SIZE set in a PDB must be less than or equal to 50% of the SGA_TARGET value at the CDB level.
If the SGA_TARGET initialization parameter is set at the PDB level, then the following requirement must be met:
- The value of DB_CACHE_SIZE set in a PDB must be less than or equal to 50% of the SGA_TARGET value at the PDB level.
If the SGA_TARGET initialization parameter is not set, but the DB_CACHE_SIZE initialization parameter is set at the CDB level, then the following requirement must be met:
- The value of DB_CACHE_SIZE set in a PDB must be less than or equal to 50% of the DB_CACHE_SIZE value at the CDB level.

When you set DB_CACHE_SIZE in a PDB to a value that does not meet these requirements, you receive an error.

本站中文解释

官方英文解释

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