RMAN

大多数操作系统支持异步I/O。

备份到磁盘，异步I/O是一个优势，因为一个服务器进程同时可以执行多个I/O操作；同步I/O必须等上一个I/O操作完成才可以执行下一个I/O操作。

初始化参数disk_asynch_io控制oracle异步I/O操作行为。如果操作系统支持异步I/O，oracle建议设置该参数为true。

如果操作系统不支持异步I/O,可以设置dbwr_io_slaves参数为非零值来调优I/O性能。（如果设置为非零值，会增加数据库进程的数量，需要增加processes的值）

如果使用的是磁带备份，查看视图v$backup_sync_io中的列effective_bytes_per_second。如果该值比磁带设备的最大吞吐小，表明磁带设备不是streaming（连续写） 。

此外，对于磁带设备，可以比较backup validate命令和实际备份任务的时间，如果时间明显比实际备份任务时间小，表明磁带的写能力很可能是瓶颈。

可以通过以下两个试图,监控异步I/O和同步I/O:

·v$backup_async_io：查看备份或者还原操作的异步I/O

·v$backup_symc_io：查看备份或者还原的同步I/O性能。

在这两个试图中，effective_bytes_per_second列的含义：Read/write rate for device during backup or restore.

备份或还原操作中，异步I/O的监控信息：

SQL> SELECT sid, serial, filename, type, elapsed_time, effective_bytes_per_second
  2  FROM v$backup_async_io
  3  WHERE close_time > sysdate-7;

       SID     SERIAL FILENAME                                                     TYPE      ELAPSED_TIME EFFECTIVE_BYTES_PER_SECOND
---------- ---------- ------------------------------------------------------------ --------- ------------ --------------------------
        53         34                                                              AGGREGATE         7500                   19028924
        53         34 /u11/app/oracle/oradata/ora11/system01.dbf                   INPUT             7500                    9521179
        53         34 /u11/app/oracle/oradata/ora11/sysaux01.dbf                   INPUT             6800                    7933832
        53         34 /u11/app/oracle/oradata/ora11/example01.dbf                  INPUT             1600                    5243392
        53         34 /u11/app/oracle/oradata/ora11/UNDOTBS02.dbf                  INPUT             1500                    4823996
        53         34 /u11/app/oracle/oradata/ora11/yb01.dbf                       INPUT              400                    2754560
        53         34 /u11/app/oracle/oradata/ora11/users01.dbf                    INPUT              200                    2363392
        53         34 /u11/app/oracle/oradata/ora11/yb02.dbf                       INPUT                0
        53         34 /u11/app/oracle/fra/ORA11/backupset/2015_07_01/o1_mf_nnndf_T OUTPUT            7500                   15325157
                      AG20150701T122458_bs6twvbn_.bkp

        53         34                                                              AGGREGATE            0
        53         34 /u11/app/oracle/product/11.2.0/dbhome_1/dbs/snapcf_ora11.f   INPUT                0
        53         34 /u11/app/oracle/fra/ORA11/autobackup/2015_07_01/o1_mf_s_8839 OUTPUT               0
                      16783_bs6tzjr5_.bkp


12 rows selected.

SQL> SELECT filename, sid, serial, close_time, long_waits/io_count as ratio
  2  FROM v$backup_async_io
  3  WHERE type != 'AGGREGATE'
  4  AND SID = &SID
  5  AND SERIAL = &SERIAL
  6  ORDER BY ratio desc;
Enter value for sid: 53
old   4: AND SID = &SID
new   4: AND SID = 53
Enter value for serial: 34
old   5: AND SERIAL = &SERIAL
new   5: AND SERIAL = 34

FILENAME                                                            SID     SERIAL CLOSE_TIM      RATIO
------------------------------------------------------------ ---------- ---------- --------- ----------
/u11/app/oracle/fra/ORA11/backupset/2015_07_01/o1_mf_nnndf_T         53         34 01-JUL-15 .249772106
AG20150701T122458_bs6twvbn_.bkp

/u11/app/oracle/fra/ORA11/autobackup/2015_07_01/o1_mf_s_8839         53         34 01-JUL-15         .2
16783_bs6tzjr5_.bkp

/u11/app/oracle/oradata/ora11/example01.dbf                          53         34 01-JUL-15          0
/u11/app/oracle/oradata/ora11/UNDOTBS02.dbf                          53         34 01-JUL-15          0
/u11/app/oracle/oradata/ora11/users01.dbf                            53         34 01-JUL-15          0
/u11/app/oracle/oradata/ora11/yb02.dbf                               53         34 01-JUL-15          0
/u11/app/oracle/product/11.2.0/dbhome_1/dbs/snapcf_ora11.f           53         34 01-JUL-15          0
/u11/app/oracle/oradata/ora11/sysaux01.dbf                           53         34 01-JUL-15          0
/u11/app/oracle/oradata/ora11/system01.dbf                           53         34 01-JUL-15          0
/u11/app/oracle/oradata/ora11/yb01.dbf                               53         34 01-JUL-15          0

10 rows selected.

SQL>