Tag Archive: smart flash cache

 

Smart Flash Cache is enabled by setting two initialization parameters. The file can be on the operating system file system, or on an ASM disk group. After instance startup, a flash cache file can be disabled by dynamically changing its size to zero. Once disabled in this manner, it can also be re-enabled by dynamically resetting its value to the original; adjusting the original value is not permitted.

db_flash_cache_file='/flash1/db1','/flash2/db1'
db_flash_cache_size=256M,128M

After instance startup, a flash cache file can be disabled by dynamically changing its size to zero. Once disabled in this manner, it can also be re-enabled by dynamically resetting its value to the original; adjusting the original value is not permitted.

-- disable /flash2
alter system set db_flash_cache_size = 256M,0;

-- re-enable /flash2
alter system set db_flash_cache_size = 256M, 256M;

When the database needs a buffer, it can overwrite a clean buffer. If this buffer is needed later, it must be read again from magnetic disk. However, if flash cache is enabled, then the buffer is written to it before being overwritten. Now if it is needed later, then it can be read from the SSD rather than magnetic disk.

Note that only the body of the block is written to SSD; the header remains in the buffer cache, and takes up about 100 bytes. In a RAC instance, it takes 200 bytes in the buffer cache, plus another 208 bytes in the shared pool for GCS.

Smart Flash Cache usage statistics are available in v$flashfilestat. For each file, it show whether the flash file is enable, and the number of reads and their latency.

select * from v$flashfilestat;

The storage clause of a table can be modified to alter the default algorithms for moving blocks from the buffer cache the flash cache.

-- keep schema object blocks in flash cache, space permitting
alter table test.flashtab storage (flash_cache keep);

-- never keep schema object blocks in flash cache
alter table test.flashtab storage (flash_cache none);

-- let database decide whether to keep schema object blocks in flash cache
alter table test.flashtab storage (flash_cache default);

 

See MOS Doc ID 2123908.1 for information about unpublished Bug 19504946 – FLASH CACHE DOESN’T WORK IN OEL7. Please apply Patch 19504946 to fix the issue. Otherwise you will get ORA-01261 during startup.