Oracle OCP认证考试题库解析052-1

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QUESTION 1


You notice that the performance of the database has degraded because of frequent checkpoints.
Which two actions resolve the issue? (Choose two.)

A. Disable automatic checkpoint tuning
B. Check the size of the redo log file size and increase the size if it is small
C. Set the FAST_START_MTTR_TARGET parameter as per the advice given by the MTTR Advisor
D. Decrease the number of redo log members if there are more than one redo log members available in each redo log group

Correct Answer: BC

Explanation/Reference:

检查点进程:

The checkpoint process (CKPT) updates the control file and data file headers with checkpoint information and signals DBWn to write blocks to disk. Checkpoint information includes the checkpoint position, SCN, location in online redo log to begin recovery, and so on. CKPT does not write data blocks to data files or redo blocks to online redo log files.

检查点作用:

(1)定期促进DBWn进程把内存的脏块写回到数据文件,数据库故障时不会丢数据

(2)减少实例恢复的时间

(3)确保所有已提交的数据在一致性关闭期间会被写入数据文件

实例恢复时间:

指的是将数据文件的最后一个检查点(检查点位置)推进到控制文件中记录的最新SCN所需的时间.管理员可以通过设置MTTR 目标以及调整重做日志组的大小来控制该时间.

MTTR:

Mean Time To Repair,即平均修复时间.是指可修复产品的平均修复时间,就是从出现故障到修复中间的这段时间.MTTR越短表示易恢复性越好.在数据库中可以通过设置参数FAST_START_MTTR_TARGET(单位为秒),控制数据库对单个实例执行崩溃恢复所花费的时间.

FAST_START_MTTR_TARGE=非0值, 启用快速启动检查点功能,自动控制发检查点速度.

FAST_START_MTTR_TARGE=0,禁用快速启动检查点功能.

如果重做日志文件的size太小,会频繁切换日志,即会频繁发生检查点,故可以增大重做日志文件的size减少检查点发生频率.

FAST_START_MTTR_TARGET如果设置的太小,为了要控制数据库单实例的实例恢复时间，则必须频繁发生检查点,确保内存中已修改的数据块能够定期写入到磁盘,故可以使用MTTR Advisor来设置推荐的值.