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