最近一直在处理新系统的性能优化问题，这两天特地测试了下oracle 11gR2开始引入的smart flash cache。
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，这很有可能会是个最大的限制，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。