rman作为oracle备份与恢复工具，为我们提供了强大的功能，其中包括对数据文件的物理和逻辑检测以及备份文件的有效性检测。 首先，来看一下rman对数据文件的检测。我们知道，rman在备份数据时，会将数据读入读缓存，然会将数据从读缓存写入写缓存并最终写入数
rman作为oracle备份与恢复工具，为我们提供了强大的功能，其中包括对数据文件的物理和逻辑检测以及备份文件的有效性检测。
首先，来看一下rman对数据文件的检测。我们知道，rman在备份数据时，会将数据读入读缓存，然会将数据从读缓存写入写缓存并最终写入数据库。在从读缓存到写缓存的过程中，rman会进行物理一致性的检测（db_block_checksum未true）和逻辑一致性检测（使用了check logical选项）。例如：
SQL> show parameter db_block NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ db_block_buffers integer 0 db_block_checking string FALSE db_block_checksum string TRUE db_block_size integer 8192 SQL>
RMAN> backup tablespace test1; Starting backup at 2014-11-06 15:12:49 using channel ORA_DISK_1 channel ORA_DISK_1: starting full datafile backupset channel ORA_DISK_1: specifying datafile(s) in backupset input datafile fno=00005 name=/home/app/oraten/oradata/oraten/test01.dbf channel ORA_DISK_1: starting piece 1 at 2014-11-06 15:12:50 RMAN-00571: =========================================================== RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS =============== RMAN-00571: =========================================================== RMAN-03009: failure of backup command on ORA_DISK_1 channel at 11/06/2014 15:12:51 ORA-19566: exceeded limit of 0 corrupt blocks for file /home/app/oraten/oradata/oraten/test01.dbf
有时，我们仅仅希望验证一下数据文件是否可以备份，而不想进行真正的备份，这是我们就可以使用validate命令了，如
RMAN> backup validate database; Starting backup at 2014-11-06 15:14:23 using channel ORA_DISK_1 channel ORA_DISK_1: starting full datafile backupset channel ORA_DISK_1: specifying datafile(s) in backupset input datafile fno=00001 name=/home/app/oraten/oradata/oraten/system01.dbf input datafile fno=00003 name=/home/app/oraten/oradata/oraten/sysaux01.dbf input datafile fno=00002 name=/home/app/oraten/oradata/oraten/undotbs01.dbf input datafile fno=00005 name=/home/app/oraten/oradata/oraten/test01.dbf input datafile fno=00004 name=/home/app/oraten/oradata/oraten/users01.dbf channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01 channel ORA_DISK_1: starting full datafile backupset channel ORA_DISK_1: specifying datafile(s) in backupset including current control file in backupset including current SPFILE in backupset channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03 Finished backup at 2014-11-06 15:14:27注意，这里并没有发现test01.dbf的物理损坏块，个人认为，rman仅仅只是将数据文件读入了读缓存，而没有进行下面的操作，也就是说对于物理块的检测其功能是有限的，但，我们可以用其来检测物理文件的可读性和位置等。
下面是官方文档中的描述：
VALIDATE Causes RMAN to scan the specified files and verify their contents, testing whether this file can be backed up. RMAN creates no output files. Use this
command periodically to check for physical and logical errors in database files.
使用validate命令可以验证备份集的有效性，官方文档：
To examine a backup set and report whether it can be restored. RMAN scans all of the backup pieces in the specified backup sets and looks at the checksums to verify that
the contents are intact so that the backup can be successfully restored if necessary. Use this command when you suspect that one or more backup pieces in a backup set
are missing or have been damaged. Use VALIDATE BACKUPSET to specify which backups to test; use the VALIDATE option of the RESTORE command to let RMAN
choose which backups to validate. For validating image copies, run RESTORE VALIDATE FROM DATAFILECOPY
示例：
RMAN> run {
2> set maxcorrupt for datafile 5 to 2;
3> backup database;}
RMAN> validate backupset 84; using channel ORA_DISK_1 channel ORA_DISK_1: starting validation of datafile backupset channel ORA_DISK_1: reading from backup piece /home/app/oraten/flash_recovery_area/ORATEN/backupset/2014_11_06/o1_mf_nnndf_TAG20141106T152820_b5p8ro7v_.bkp channel ORA_DISK_1: restored backup piece 1 piece handle=/home/app/oraten/flash_recovery_area/ORATEN/backupset/2014_11_06/o1_mf_nnndf_TAG20141106T152820_b5p8ro7v_.bkp tag=TAG20141106T152820 channel ORA_DISK_1: validation complete, elapsed time: 00:00:01validate命令验证的是备份的物理有效性而不是存储在备份中的数据块的物理有效性。
RESTORE ... PREVIEW
该命令进行restore命令的预演，可以在任何resotre命令后加上preview来进行还原预演，这个命令只是从数据字典读出需要的备份的元数据，不进行文件的验证。
You can apply RESTORE ... PREVIEW to any RESTORE operation to create a detailed list of every backup to be used in the requested RESTORE operation, as well as the necessary target SCN for recovery after the RESTORE operation is complete. This command accesses the RMAN repository to query the backup metadata, but does not actually read the backup files to ensure that they can be restored.
RMAN> restore database preview;
Starting restore at 2014-11-06 15:44:28
using channel ORA_DISK_1
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ -------------------
84 Full 551.76M DISK 00:00:18 2014-11-06 15:28:24
BP Key: 52 Status: AVAILABLE Compressed: NO Tag: TAG20141106T152820
Piece Name: /home/app/oraten/flash_recovery_area/ORATEN/backupset/2014_11_06/o1_mf_nnndf_TAG20141106T152820_b5p8ro7v_.bkp
List of Datafiles in backup set 84
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- ------------------- ----
1 Full 922948 2014-11-06 15:28:20 /home/app/oraten/oradata/oraten/system01.dbf
2 Full 922948 2014-11-06 15:28:20 /home/app/oraten/oradata/oraten/undotbs01.dbf
3 Full 922948 2014-11-06 15:28:20 /home/app/oraten/oradata/oraten/sysaux01.dbf
4 Full 922948 2014-11-06 15:28:20 /home/app/oraten/oradata/oraten/users01.dbf
5 Full 922948 2014-11-06 15:28:20 /home/app/oraten/oradata/oraten/test01.dbf
archive logs generated after SCN 922948 not found in repository
Media recovery start SCN is 922948
Recovery must be done beyond SCN 922948 to clear data files fuzziness
Finished restore at 2014-11-06 15:44:28
RECOVER ... TEST：在内存中模拟recover过程，并不进行真正的recovery，可以用该命令来提起发现recovery中的问题。注意，模拟的recovery和正常recovery可能会遇到不同的问题。A simulated recovery initiated with the RECOVER ... TEST command in RMAN or SQL*Plus. A trial recovery applies redo in a way similar to normal media recovery, but it never writes its changes to disk and it always rolls back its changes. Trial recovery occurs only in memory.
Trial recovery lets you foresee what problems might occur if you were to continue with normal recovery. For problems caused by ongoing memory corruption, trial recovery and normal recovery can encounter different errors.
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