[每日一题] 11gOCP 1z0-052 :2013-09-13 FAST_START_MTTR_TARGET.........................................A69

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答案　BC

sys@OCM> alter system set fast_start_mttr_target=0;   ----显示设为0

答案A不正确，这两参数与SGA顾问器无关。

答案B:正确，当fast_start_mttr_target 默认为0（即没有手工设0）说明是开启了自动检查点，（fast_start_mttr_target=0或默认，mttr顾问器都会被禁用）。

答案C正确，显式设置FAST_START_MTTR_TARGET = 0，自动检查点调整被禁用了（来自MOS:Automatic checkpoint tuning is enabled by default. If it is disabled, by setting the parameter to zero explicitly）

Automatic Checkpoint Tuning in Oracle 10g (文档 ID 265831.1)

APPLIES TO:

Oracle Server - Enterprise Edition - Version: 10.1.0.2 to 11.2.0.3 - Release: 10.1 to 11.2
Information in this document applies to any platform.
***Checked for relevance on 10-Oct-2011***

PURPOSE

To understand the new feature of "Automatic Checkpoint Tuning" introduced in Oracle 10g.

SCOPE

The note is intended for DBA's to get quickly familiar with this new Oracle 10g feature.

AUTOMATIC CHECKPOINT TUNING IN ORACLE 10G

Automatic Checkpoint Tuning

Oracle Database 10g supports automatic checkpoint tuning. It is enabled if fast_start_mttr_target is explicitly set to an non-zero value, or if fast_start_mttr_target is not set at all. 

It is an advancement over the MTTR related parameter introduced in earlier versions. The idea is to use the periods of low I/O usage to advance checkpoints and therefore improve availability.

How to enable

Automatic checkpoint tuning is enabled by default. If it is disabled, by setting the parameter to zero explicitly, you can enable it by unsetting FAST_START_MTTR_TARGET or set it to a nonzero value. If you set this parameter to zero this feature will be disabled. 
Note: this is different from defaulting (i.e. not setting) fast_start_mttr_target to 0. 

It can be done statically using the initialization files or dynamically using 

SQL> alter system set FAST_START_MTTR_TARGET=10;

Note: This is measured in seconds 

When to enable

• system I/O capacity is insufficient with fast-start checkpointing
• If it is necessary to control the time to recover from an instance or node failure, then set FAST_START_MTTR_TARGET to the desired MTTR in seconds.

How it works

Enabling fast-start checkpointing increases the average number of writes per transaction that DBWn issues for a given workload. However, if the system is not already near or at its maximum I/O capacity, then 
fast-start checkpointing has a negligible impact on performance 

How to monitor

View V$MTTR_TARGET_ADVICE will give information on the additional I/O's on various values of FAST_START_MTTR_TARGET. 

PROs and CONs

• FAST_START_MTTR_TARGET is set to a low value:
  Fast-start checkpointing is more aggressive. The average number of writes per transaction that DBWn issues is higher in order to keep the thread checkpoint sufficiently advanced to meet the requested MTTR
  
  
• FAST_START_MTTR_TARGET is set to a high value:
  Fast-start checkpointing in less aggressive, and the average number of writes per transaction that DBWn issues is lower.
  
  In both cases above automatic checkpoint is enabled.
  
  
• FAST_START_MTTR_TARGET is unset:
  Automatic checkpoint tuning is in effect. Average number of writes per transaction is reduced but at the same time MTTR is highest.

NOTE:

1. When you enable fast-start checkpointing, remove or disable (set to 0) the following initialization parameters:
   • LOG_CHECKPOINT_INTERVAL
   • LOG_CHECKPOINT_TIMEOUT
   • FAST_START_IO_TARGET
     
     
2. Automatic Checkpoint tuning using FAST_START_MTTR_TARGET is only available with the Oracle Enterprise Edition and is also referred to as 'Fast-start fault recovery'.
   Please refer to
     Oracle® Database Licensing Information
     11g Release 2 (11.2)
     Part Number E10594-18
     http://download.oracle.com/docs/cd/E11882_01/license.112/e10594/editions.htm#CJACGHEB

1、默认是开启的自动检查点！！！！，即fast_start_mttr_target没做任何设。

sys@OCM> show parameter mttr

 

NAME                                TYPE        VALUE

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

fast_start_mttr_target              integer     0

 

隐藏参数“_disable_selftune_checkpointing”（Disable self-tune checkpointing ）＝                   FALSE 

    

sys@OCM> show parameter dump

 

NAME                                TYPE        VALUE

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

background_core_dump                string      partial

background_dump_dest                string     /u01/app/oracle/diag/rdbms/ocm

                                                /ocm/trace

core_dump_dest                      string     /u01/app/oracle/diag/rdbms/ocm

                                                /ocm/cdump

max_dump_file_size                  string      unlimited

shadow_core_dump                    string      partial

user_dump_dest                      string     /u01/app/oracle/diag/rdbms/ocm

                                                /ocm/trace

 

[root@mydb ~]# su - oracle

[oracle@mydb ~]$ more /u01/app/oracle/diag/rdbms/ocm/ocm/trace/

 

ARC1 started with pid=25, OS id=10803

Thread 1 opened at log sequence 12

  Current log# 3 seq# 12 mem# 0:/u01/app/oracle/oradata/ocm/redo03.log

Successful open of redo thread 1

MTTR advisory is disabled because FAST_START_MTTR_TARGET is not set

Sun Sep 08 14:38:03 2013

ARC2 started with pid=26, OS id=10805

2、如果说显式的设置alter system set FAST_START_MTTR_TARGET=0就是关闭自动调节。

idle> alter system set fast_start_mttr_target=0;


System altered.


sys@OCM> startup force;
ORACLE instance started.


Total System Global Area  839282688 bytes
Fixed Size                  2233000 bytes
Variable Size             515902808 bytes
Database Buffers          314572800 bytes
Redo Buffers                6574080 bytes
Database mounted.
Database opened.

[root@mydb trace]# vi alert_ocm.log 

MTTR advisory is disabled because FAST_START_MTTR_TARGET is not set

MTTR advisory was turned off because FAST_START_MTTR_TARGET was disabled.

3、当FAST_START_MTTR_TARGET显示设为非零

idle> alter system set fast_start_mttr_target=120;


System altered.


idle> startup force;
ORACLE instance started.


Total System Global Area  839282688 bytes
Fixed Size                  2233000 bytes
Variable Size             515902808 bytes
Database Buffers          314572800 bytes
Redo Buffers                6574080 bytes
Database mounted.
Database opened.

[root@mydb trace]# vi alert_ocm.log 

当FAST_START_MTTR_TARGET显示设为非0，没发现MTTR advisory 相关的信息。

总结：FAST_START_MTTR_TARGET默认或显示设为在alert日志中都会有MTTR advisory is disabled。

MOS Automatic Checkpoint Tuning in Oracle 10g [ID 265831.1]

答案D不正确的，跟这个参数有关，如果设为ture就会把检查点信息写到aler日志

sys@OCM> show parameter alert

 

NAME                                TYPE        VALUE

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

log_checkpoints_to_alert            boolean     FALSE

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