[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

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