You can find an overview of the feature in the Oracle Database Administrator's Guide. Basically, if you have flash devices attached to your server, you can use this flash memory to increase the size of the buffer cache. So instead of aging blocks out of the buffer cache and having to go back to reading them from disk, they move to the much, much faster flash storage as a secondary fast buffer cache (for reads, not writes).
Some scenarios where this is very useful : you have huge tables and huge amounts of data, a very, very large database with tons of query activity (let's say many TB) and your server is limited to a relatively small amount of main RAM - (let's say 128 or 256G). In this case, if you were to purchase and add a flash storage device of 256G or 512G (example), you can attach this device to the database with the Database Smart Flash Cache feature and increase the buffercache of your database from like 100G or 200G to 300-700G on that same server. In a good number of cases this will give you a significant performance improvement without having to purchase a new server that handles more memory or purchase flash storage that can handle your many TB of storage to live in flash instead of rotational storage.
It is also incredibly easy to configure.
-1 install Oracle Linux (I installed Oracle Linux 6 with UEK3)
-2 install Oracle Database 12c (this would also work with 11g - I installed 12.1.0.2.0 EE)
-3 add a flash device to your system (for the example I just added a 1GB device showing up as /dev/sdb)
-4 attach the storage to the database in sqlplus
Done.
$ ls /dev/sdb/dev/sdb$ sqlplus '/ as sysdba'SQL*Plus: Release 12.1.0.2.0 Production on Tue Feb 24 05:46:08 2015Copyright (c) 1982, 2014, Oracle. All rights reserved.Connected to:Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit ProductionWith the Partitioning, OLAP, Advanced Analytics and Real Application Testing optionsSQL>alter system set db_flash_cache_file='/dev/sdb' scope=spfile;System altered.SQL>alter system set db_flash_cache_size=1G scope=spfile;System altered.SQL>shutdown immediate;Database closed.Database dismounted.ORACLE instance shut down.SQL>startupORACLE instance started.Total System Global Area 4932501504 bytesFixed Size
2934456 bytesVariable Size 1023412552 bytesDatabase Buffers 3892314112 bytesRedo Buffers 13840384 bytesDatabase mounted.Database opened.SQL>show parameters flashNAME TYPE VALUE------------------------------------ ----------- ------------------------------db_flash_cache_file string /dev/sdbdb_flash_cache_size big integer 1Gdb_flashback_retention_target integer 1440SQL>select * from v$flashfilestat; FLASHFILE#----------NAME-------------------------------------------------------------------------------- BYTES ENABLED SINGLEBLKRDS SINGLEBLKRDTIM_MICRO CON_ID---------- ---------- ------------ -------------------- ---------- 1/dev/sdb1073741824 1 0 0 0You can get more information on configuration and guidelines/tuning here.If you want selective control of which tables can use or will use the Database Smart Flash Cache, you can use the ALTER TABLE command. See here. Specifically the STORAGE clause. By default, the tables are aged out into the flash cache but if you don't want certain tables to be cached you can use the NONE option. alter table foo storage (flash_cache none);This feature can really make a big difference in a number of database environments and I highly recommend taking a look at how Oracle Linux and Oracle Database 12c can help you enhance your setup. It's included with the database running on Oracle Linux.
Here is a link to a white paper that gives a bit of a performance overview.