Oracle 参数 DB_FLASH_CACHE_SIZE 官方解释，作用，如何配置最优化建议
本站中文解释
DB_FLASH_CACHE_SIZE参数的功能：
DB_FLASH_CACHE_SIZE参数是控制数据库闪存缓存的大小的一个参数。通过设置这个参数，它可以把闪存缓存的大小设置在允许的范围内，以便可以有效地使用闪存缓存来提高数据库性能。
正确设置DB_FLASH_CACHE_SIZE参数：
1.首先，在设置这个参数之前，您需要确定您的数据库是否有可用的闪存缓存。如果您的数据库不允许使用闪存缓存空间，则设置这个参数是没有意义的；
2.其次，你需要确定可以分配的最大闪存缓存空间的大小。闪存缓存的大小受物理内存的限制，因此首先要确定有多少内存可将分配给闪存缓存；
3.最后，当您确定了可以分配多少闪存缓存时，您可以使用DB_FLASH_CACHE_SIZE参数来设置闪存缓存的最大值。按照官方文档，其有效范围为0-25GB。
官方英文解释
DB_FLASH_CACHE_SIZE specifies the size of the Database Smart Flash Cache (flash cache). This parameter may only be specified at instance startup.
| Property | Description |
|---|---|
|
Parameter type |
Big integer |
|
Syntax |
|
|
Default value |
0 |
|
Modifiable |
|
|
Modifiable in a PDB |
No |
|
Range of values |
0, 16M to (2048G – DB_BLOCK_SIZE) |
|
Basic |
No |
You can specify up to 16 file sizes, for each of the flash memory devices specified with DB_FLASH_CACHE_FILE. For example, if there are three flash raw devices, you can specify the sizes of each device as follows:
db_flash_cache_file = /dev/raw/sda, /dev/raw/sdb, /dev/raw/sdc db_flash_cache_size = 32G, 32G, 64G
If your flash cache consists of one flash cache device, you can dynamically change this parameter to 0 for that flash cache device (disabling the flash cache) after the database is started. You can then reenable the flash cache by setting this parameter for the device back to the original value when the database was started. Dynamic resizing of DB_FLASH_CACHE_SIZE or reenabling flash cache to a different size is not supported.
If your flash cache includes multiple flash cache devices, you can dynamically change the parameter to 0 for a particular flash cache device (turning it off) after the database is started. You can then reenable that flash cache device by setting this parameter for the device back to the original value it had when the database was started (turning it back on).
For example, to turn off the /dev/raw/sdb flash cache device:
db_flash_cache_file = /dev/raw/sda, /dev/raw/sdb, /dev/raw/sdc db_flash_cache_size = 32G, 0, 64G
And, to turn the /dev/raw/sdb flash cache device back on again:
db_flash_cache_file = /dev/raw/sda, /dev/raw/sdb, /dev/raw/sdc db_flash_cache_size = 32G, 32G, 64G
See Also:
“DB_FLASH_CACHE_FILE”
