参数FAST_START_MTTR_TARGET

一、关于FAST_START_MTTR_TARGET参数

是一个加快实例恢复的参数，我们可以根据服务级别来定义一个合理的、可接受的值，该值的单位为秒。比如设定为60s，即1分钟。

假定该值处于合理的情况之下，则一旦实例崩溃，在60s以内实例应当能够被恢复。合理即是该值不能太大，也不能太小。太大则实例恢复所需的时间较长，太小则导致大量数据的及时写入，增加了系统的I/O。

影响实例恢复时间长短的主要因素即是从最近检查点位置到联机重做日志尾部之间的距离。距离越长则所需要的cache recovery 和undo、redo的时间越长。所以如何有效的缩短最近检查点位置与联机重做日志尾部之间的距离，这正是FAST_START_MTTR_TARGET的目的。

关于检查点的触发条件有很多，比如日志切换、数据库shutdown、开始结束备份表空间等。检查点的分类也很多，比如完全检查点、部分检查点、增量检查点等。

FAST_START_MTTR_TARGET的值实际上也是触发检查点的一个触发条件。当内存中产生的dirty buffer所需的恢复时间(estimated_mttr)到达FAST_START_MTTR_TARGET的指定时间，则检查点进程被触发。检查点进程一旦被触发，将通知DBWn进程将按检查点队列顺序将脏数据写入到数据文件，从而缩短了最后检查点位置与联机重做日志间的距离，减少了实例恢复所需的时间。

二、FAST_START_MTTR_TARGET =
0的质疑

很多文章描述了FAST_START_MTTR_TARGET
=
0，即为未设置，表示启用自动检查点功能，下面是来自Oracle的官方文档中的一段，

原文的链接为：Fast-Start
Fault Recovery

Fast-start checkpointing refers to the periodic writes by the
database writer (DBWn) processes for the purpose of
writing changed data blocks from the Oracle
buffer cache to disk and advancing the thread-checkpoint.
Setting the database
parameter FAST_START_MTTR_TARGET to a value
greater than zero enables the fast-start checkpointing
feature. Fast-start checkpointing
should always be enabled for the following
reasons:

It reduces the time required for cache recovery, and makes instance
recovery time-bounded and predictable. This is accomplished
by limiting the number of dirty
buffers (data blocks which have changes in memory that still need
to be written to disk) and the number of redo records (changes in
the database) generated between the most recent redo record and the
last checkpoint.

Fast-Start checkpointing eliminates bulk writes and corresponding
I/O spikes that occur traditionally with interval-
based checkpoints, providing a smoother,
more consistent I/O pattern that is more predictable and easier to
manage. If the system is not already near or at its
maximum I/O capacity, fast-start checkpointing will have a
negligible impact on performance. Although
fast-start checkpointing results in
increased write activity, there is little reduction in database
throughout, provided the system has sufficient
I/O capacity.

从第一段中粗体标记的描述来看，当设定一个大于0的值给FAST_START_MTTR_TARGET，则自动调整检查点功能别启用。即fast-start checkpointing，更准确的说应该是快速启动检查点功能

再看下面的这段描述，这段来自Oracle 10g OCP
workshop I 14-17
英文版教程(Edition 3.1
December 2008)

Explicit setting of
the FAST_START_MTTR_TARGET parameter to 0 disables automatic
checkpoint tuning.Explicit setting of
the FAST_START_MTTR_TARGET parameter to a
value other than 0 also enables the Redo Log
Advisor.

从上面的描述可以看出，如果将FAST_START_MTTR_TARGET设置为0将关闭检查点自动调整功能。

三、设定FAST_START_MTTR_TARGET

根据实际需要来设定FAST_START_MTTR_TARGET的值，这个值的设定需要考虑到可接受的实例的恢复时间、可承受的I/O吞吐量等等。

假定我们将该值设定为

SQL> alter system set fast_start_mttr_target = 30 ;

在事务频繁的时间段来考察视图v$instacne_recovery提供的值

SQL> desc v$instance_recovery;
--查看v$instance_recovery视图的结构

Name
Null?
Type

----------------------------------------- --------
----------------------------

RECOVERY_ESTIMATED_IOS
NUMBER

ACTUAL_REDO_BLKS
NUMBER

TARGET_REDO_BLKS
NUMBER

LOG_FILE_SIZE_REDO_BLKS
NUMBER

LOG_CHKPT_TIMEOUT_REDO_BLKS
NUMBER

LOG_CHKPT_INTERVAL_REDO_BLKS
NUMBER

FAST_START_IO_TARGET_REDO_BLKS
NUMBER

TARGET_MTTR
NUMBER

ESTIMATED_MTTR
NUMBER

CKPT_BLOCK_WRITES
NUMBER

OPTIMAL_LOGFILE_SIZE
NUMBER

ESTD_CLUSTER_AVAILABLE_TIME
NUMBER

WRITES_MTTR
NUMBER

WRITES_LOGFILE_SIZE
NUMBER

WRITES_LOG_CHECKPOINT_SETTINGS
NUMBER

WRITES_OTHER_SETTINGS
NUMBER

WRITES_AUTOTUNE
NUMBER

WRITES_FULL_THREAD_CKPT
NUMBER

两个字段：

TARGET_MTTR
-->参照fast_start_mttr_target参数中设定的值计算出来的一个值

ESTIMATED_MTTR
-->系统根据dirty buffer 中计算出来的值

可能出现的情况

1.TARGET_MTTR >
ESTIMATED_MTTR --大量的事务将导致这种情况的出现（存疑-特此标记）

2.TARGET_MTTR <
ESTIMATED_MTTR --数据库刚刚启动时，几乎没有事务时会出现这种情况（存疑-特此标记）

SQL> select recovery_estimated_ios,actual_redo_blks
,target_redo_blks ,
2
target_mttr,estimated_mttr
3 from v$instance_recovery;

RECOVERY_ESTIMATED_IOS
ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR
ESTIMATED_MTTR
----------------------
---------------- ---------------- -----------
--------------

55
147
707
33
27

可以在负载的情况下根据TARGET_MTTR的值，通过v$mttr_target_advice调整该参数

四、启用MTTR
Advisory

需要设置两个参数

STATISTICS_LEVEL
-->置为typical 或者all

FAST_START_MTTR_TARGET
-->置为非零值

SQL> show parameter
mttr;
--目标mttr_time设定为30 s

NAME
TYPE
VALUE

------------------------------------
----------- ------------------------------

fast_start_mttr_target
integer
30

SQL> select target_mttr,estimated_mttr
from
v$instance_recovery;
--系统计算出来的mttr为33

TARGET_MTTR ESTIMATED_MTTR

-----------
--------------

33
27

SQL> select mttr_target_for_estimate tar_est,dirty_limit,estd_cache_writes est_c_w,

2
estd_cache_write_factor
est_c_w_f,estd_total_writes est_t_w,estd_total_write_factor est_t_w_f

3
from v$mttr_target_advice;

TAR_EST DIRTY_LIMIT
EST_C_W
EST_C_W_F
EST_T_W
EST_T_W_F

---------- ----------- ----------
---------- ---------- ----------

60
5028
3762
.7376
3762
.7376

34
1000
5100
1
5100
1

68
6248
3762
.7376
3762
.7376

52
3808
3762
.7376
3762
.7376

45
2735
3845
.7539
3845
.7539

--mttr_target_for_estimate有一个值为的最接近设定的目标时间30，以及由系统计算出的的target_mttr时间33

--同时也给出了几组不同的mttr_target值及dirty_limit,cache_write,io 等来供DBA来选择设定合适的mttr

五、两个重要的视图

v$instacne_recovery

v$mttr_target_advice

转自：http://blog.csdn.net/robinson_0612/article/details/5782516