[bbk5378] 第89集 第11章 数据库诊断 05

oracle在内存中操作数据块，达到一定条件后，往磁盘文件上去读或者写的时候，会检查是否存在坏块.

db_block_checksum参数设置成off的时候，Oracle只针对系统表空间的数据块进行检查，非系统表空间则不关心.

假如db_block_checksum=typical，oracle会在由内存写到磁盘的时候，会检查是否存在坏块.

假如db_block_checksum=full，oracle会在执行DML操作之前，都会检查是否存在坏块.不管是内存中的还是磁盘上的块,只要发生insert，delete，update发生多少次就会检查多少次，如果是内存中的，当往磁盘中写的时候还要检查，提高了数据的安全性，牺牲了数据库性能能.毕竟安全第一嘛，还是值得的.不过oracle推荐值，默认值是typical。

解读db_lost_write_protect参数:

　　none，表空间丢失写保护的功能没有打开

　　typical，对于可读可写的表空间，其丢失写保护的功能打开.

　　full,对于只读性的表空间，其丢失数据写保护的功能打开.

假如想实现数据库块的恢复，首先需要识别出哪些块坏了，坏在哪个表空上，哪个数据文件上,要实现上述要求，首先需要设置Oracle初始化参数，来开启此项功能,具体设置要求如下:

DB_ULTRA_SAFE	OFF	DATA_ONLY	DATA_AND_INDEX
DB_BLOCK_CHECKING	OFF or FALSE	MEDIUM	FULL or TRUE
DB_BLOCK_CHECKSUM	TYPICAL	FULL	FULL
DB_BLOCK_WRITE_PROTECT	TYPICAL	TYPICAL	TYPICAL

表功能性解读:当DB_ULTRA_SAFE设置为OFF时，DB_BLOCK_CHECKING自动设置为OFF or FALSE

　　　　　　　　　　　　　　　　　　　　　　　 DB_BLOCK_CHECKSUM自动设置为TYPICAL

　　　　　　　　　　　　　　　　　　　　　　　 DB_BLOCK_WRITE_PROTECT自动设置为TYPICAL

Block Media Recovery

• Lowers the mean time to recover(MTTR)
• Increases availability during media recovery
  • -The data file remains online during recovery
  • -Only blocks being recovered are inaccessible
• Is invoked using the RMAN RECOVER...BLOCK command
  • Restores blocks using flashback logs and full or level 0 backups
  • Media recovery is performed using redo logs
• The V$DATABASE_BLOCK_CORRUPTION view displays blocks marked corrupt.

在进行坏块恢复的时候，数据库处于Online状态,其他无损的块任然可以正常访问，只是修复坏块；如此以来提高了数据库的可访问性.　　　　

Prerequisites for Block Media Recovery(块媒介恢复的先决条件)

• The target database must be in ARCHIVELOG mode.
• The backups of the data files containing the corrupt blocks must be full or level 0 backups
  • -Proxy copies must be restored to a non-default location before they can be used.
• RMAN can use only archived redo logs for the recovery.
• The corrupted data block can be restored form Flashback logs if available.

The RECOVER...BLOCK Command

The RMAN RECOVER...BLOCK command:

• Identifies the backups containing the blocks to recover
• Reads the backups and accumulates requested blocks into in-memory buffers
• Manages the block media recovery session by reading the archive logs from backup if necessary.

RECOVER DATAFILE 6 BLOCK 3;--Recover a single block



RECOVER　　　　　　　　--Recover multiple blocks in multiple data files
DATAFILE 2 BLOCK 43

DATAFILE 2 BLOCK 79

DATAFILE 6 BLOCK 183;



RECOVER CORRUPTION LIST; --Recover all blocks logged in V$DATABASE_BLOCK_CORRUPTION