其介绍参考MOS文档，How To Size the Database Smart Flash Cache (文档 ID 1317950.1)
The Database Smart Flash Cache is a new feature in Oracle Database 11g Release 2 (11.2).
The Database Smart Flash Cache is a transparent extension of the database buffer cache using solid state device (SSD) technology.
The SSD acts as a Level 2 cache to the (Level 1) SGA.
Database Smart Flash Cache can greatly improve the performance of Oracle databases by reducing the amount of disk I/O at a much lower cost than adding an equivalent amount of RAM.
简单的说，他比较适合于系统中绝大部分存储使用机械硬盘，但是又配备了小部分SSD的场景，比如说我们的某个系统4.5T存储，600GB的SSD。
Your database is running on the Solaris or Oracle Linux operating systems.
The flash cache is supported on these operating systems only.
如果不是solaris或者OEL，则启动时报错，这很有可能会是个 大的限制，因为很多企业限定了必须使用RHEL/CENTOS/SUSE。如下：
SQL> startup;
ORA-00439: feature not enabled: Server Flash Cache
这明显就是Oracle设置的障碍。
可以通过DB_FLASH_CACHE_FILE和db_flash_cache_size设置智能闪存的位置以及大小。
下面来看实际效果：
[oracle@oel-12c ~]$ sqlplus "/as sysdba"
SQL*Plus: Release 12.2.0.1.0 Production on 星期五 9月 7 20:13:00 2018
Copyright (c) 1982, 2016, Oracle. All rights reserved.
连接到:
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production
SQL> create tablespace my_ts_2 datafile '/u01/app/oracle/oradata/nfs_to_14/my_ts02.dbf' size 1g autoextend on next 10m maxsize unlimited;
表空间已创建。
SQL> create table my_big_table as select * from dba_tables;
表已创建。
SQL> alter table my_big_table move tablespace my_ts_2; --移动到NFS存储上
表已更改。
SQL> insert into my_big_table select * from my_big_table;
已创建 2106 行。
SQL> /
已创建 4212 行。
SQL> /
已创建 8424 行。
SQL> /
已创建 16848 行。
SQL> /
已创建 33696 行。
SQL> /
已创建 67392 行。
SQL> commit;
提交完成。
SQL> /
提交完成。
SQL> insert into my_big_table select * from my_big_table;
已创建 134784 行。
SQL> /
已创建 269568 行。
SQL> commit;
提交完成。
SQL> insert into my_big_table select * from my_big_table;
已创建 539136 行。
SQL> commit;
使用智能闪存：
SQL> set autotrace on;
SQL> select count(1) from my_big_table;
COUNT(1)
----------
1078272
已用时间: 00: 00: 33.06
执行计划
----------------------------------------------------------
Plan hash value: 1307946652
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12973 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| MY_BIG_TABLE | 969K| 12973 (1)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
统计信息
----------------------------------------------------------
124 recursive calls
0 db block gets
47210 consistent gets
47193 physical reads
132 redo size
546 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
3 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
COUNT(1)
----------
1078272
已用时间: 00: 00: 03.00
执行计划
----------------------------------------------------------
Plan hash value: 1307946652
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12973 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| MY_BIG_TABLE | 969K| 12973 (1)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
47094 consistent gets
47006 physical reads
0 redo size
546 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
COUNT(1)
----------
1078272
已用时间: 00: 00: 04.54
执行计划
----------------------------------------------------------
Plan hash value: 1307946652
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12973 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| MY_BIG_TABLE | 969K| 12973 (1)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
47094 consistent gets
47020 physical reads
0 redo size
546 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
COUNT(1)
----------
1078272
已用时间: 00: 00: 02.24
不使用智能闪存：
SQL> alter system set db_flash_cache_size=0 ;
系统已更改。
已用时间: 00: 00: 01.01
SQL> select count(1) from my_big_table;
COUNT(1)
----------
1078272
已用时间: 00: 00: 30.75
执行计划
----------------------------------------------------------
Plan hash value: 1307946652
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12973 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| MY_BIG_TABLE | 969K| 12973 (1)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
47094 consistent gets
47020 physical reads
0 redo size
546 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
COUNT(1)
----------
1078272
已用时间: 00: 00: 32.20
执行计划
----------------------------------------------------------
Plan hash value: 1307946652
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12973 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| MY_BIG_TABLE | 969K| 12973 (1)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
47094 consistent gets
47020 physical reads
0 redo size
546 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
COUNT(1)
----------
1078272
已用时间: 00: 00: 17.56
执行计划
----------------------------------------------------------
Plan hash value: 1307946652
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12973 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| MY_BIG_TABLE | 969K| 12973 (1)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
47094 consistent gets
47019 physical reads
0 redo size
546 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
COUNT(1)
----------
1078272
已用时间: 00: 00: 15.11
执行计划
----------------------------------------------------------
Plan hash value: 1307946652
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12973 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| MY_BIG_TABLE | 969K| 12973 (1)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
47094 consistent gets
47019 physical reads
0 redo size
546 bytes sent via SQL*Net to client
607 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
COUNT(1)
----------
1078272
已用时间: 00: 00: 15.69
如上所述，性能相差了6-7倍。可见智能闪存效果还是不错的。
不过需要注意的是，如果95%+的常用数据都已经在iops足够高的磁盘上了比如SSD，这个时候又拿一部分SSD作为智能缓存，性能反而会下降比直接访问磁盘高达1倍。
在oracle 11g中，闪存文件只能配置1个，在12c中没有这个限制了。