Effect of a Backup Retention Policy on Backups of Backups

xiangni330

看到11.1官方文档中有如下一段话，红色的字没有看懂，求解答，谢谢。
For the purposes of a backup retention policy based on redundancy, a backup set is counted as one instance of a backup. This statement is true even if there are multiple copies of the backup pieces that make up backup set, such as when a backup set has been backed up from disk to tape.
For the purposes of a recovery window retention policy, either all of the copies of a backup set are obsolete, or none of them are.
This point is easiest to grasp when viewing the output of the LIST and REPORT commands.
To view the effect of a backup retention policy on backups of backups:

Back up a datafile.
The following example backs up datafile 5:
BACKUP AS BACKUPSET DATAFILE 5;
Run the LIST command for the datafile backup in the preceding step.
For example, run the following command (sample output included).
LIST BACKUP OF DATAFILE 5 SUMMARY;

List of Backups
===============
Key     TY LV S Device Type Completion Time #Pieces #Copies Compressed Tag
------- -- -- - ----------- --------------- ------- ------- ---------- ---
18      B  F  A DISK        04-AUG-07       1       1       NO         TAG20070804T160 134
Use the backup set key from the previous step to back up the backup set.
For example, enter the following command:
BACKUP BACKUPSET 18;
Run the same LIST command that you ran in step 2.
For example, run the following command (sample output included).
LIST BACKUP OF DATAFILE 5 SUMMARY;

List of Backups
===============
Key     TY LV S Device Type Completion Time #Pieces #Copies Compressed Tag
------- -- -- - ----------- --------------- ------- ------- ---------- ---
18      B  F  A DISK        04-AUG-07       1       2       NO         TAG20070804T160 134
Only one backup set is shown in this output, but there are now two copies of it.
Generate a report to see the effect of these copies under a redundancy-based backup retention policy.
For example, issue the following command:
REPORT OBSOLETE REDUNDANCY 1;

None of the copies is reported as obsolete because both copies of the backup set have the same values for set_stamp and set_count.
Generate a report to see the effect of these copies under a recovery window-based backup retention policy.
For example, issue the following command:
REPORT OBSOLETE RECOVERY WINDOW 1 DAY;

None of the copies of the backup set is reported as obsolete or based on the CHECKPOINT_CHANGE# of this backup set, with respect to the current time and the availability of other backups.

Maclean Liu(刘相兵

这里 文档时想解释 backup policy中： redundancy和 recovery windows的区别

A retention Policy must be defined -redundancy or recovery Window


保留策略描述哪些备份将被保留以及保留时间。
• 有以下两种类型的保留策略：
– 恢复窗口：建立一个时间段，在该时段内必须可以进行
时间点恢复
– 冗余：建立必须保留的固定数量的备份
• 这些策略互相排斥，并且都可以使用CONFIGURE
命令进行设置


recovery Window 是一种保留策略

保留策略
保留策略描述哪些备份将被保留以及保留时间。可通过CONFIGURE 命令设置保留策略的
值。最佳方案是确定一个时段，在此期间内可以发现逻辑错误，然后通过执行时间点恢复
正好恢复到错误前的那一点来修复受影响的对象。这段时间期称为恢复窗口。此策略用天
数指定。对于每个数据文件，都必须始终存在一个满足以下条件的备份：
SYSDATE - checkpoint_time <= recovery_window
例如，如果策略设置如下：
RMAN> CONFIGURE RETENTION POLICY
2 TO RECOVERY WINDOW OF 7 DAYS;
那么，每个文件必须有一个满足以下条件的备份：
SYSDATE - (SELECT checkpoint_time
FROM V$DATAFILE WHERE file#= …) >= 7

应保持恢复窗口时间期小于或等于控制文件参数CONTROL_FILE_RECORD_KEEP_TIME
的值，以防止控制文件中较早备份的记录被覆盖。


如果需要保留一定数量的备份，可以通过冗余选项设置保留策略。此选项要求在任何备份
被标识为过时之前将指定数量的备份列入目录。默认保留策略的冗余度为1，这表示在任
意指定时间一个文件只存在一个备份。如果已经备份了相同文件的较新版本，那么该文件
原来的备份将视为作废。

xiangni330

谢谢Maclean Liu的解答。
redundancy和 recovery windows的区别我知道，我帖子上说的应该是备份集的备份（backup backupset），不是普通的备份在redundancy和 recovery windows rentention policy的不同,但是我没有看懂，还请你再为我解答一下：
For the purposes of a backup retention policy based on redundancy, a backup set is counted as one instance of a backup. This statement is true even if there are multiple copies of the backup pieces that make up backup set, such as when a backup set has been backed up from disk to tape.
For the purposes of a recovery window retention policy, either all of the copies of a backup set are obsolete, or none of them are.

None of the copies is reported as obsolete because both copies of the backup set have the same values for set_stamp and set_count.
Generate a report to see the effect of these copies under a recovery window-based backup retention policy.

None of the copies of the backup set is reported as obsolete or based on the CHECKPOINT_CHANGE# of this backup set, with respect to the current time and the availability of other backups.

wind

这个帖子不是问redundancy和 recovery windows的区别，是问backup backupset在两个策略里怎么算。

backup backupset例子里，是对备份集的再一次备份，其实例子也说了，其实就是一个备份，两个copy，拷贝了两次。

你的那段红字是说，（backup backupset）两个copy，算作一次备份，针对retundancy policy来说，譬如你的retundancy policy是1，就是说（backup backupset）两个copy当做一次备份，都不会过期。这个例子是我举的，这样你更好理解，鬼佬的思维写的有点繁琐。

（backup backupset）针对时间窗口来说，他们有相同的时间和备份次数，此时例子里也是没有过期的。但是他们两个copy要么都过期，要么都不过期。

不知道我解释清楚了没有。不明白再问。

Maclean Liu(刘相兵

a backup set is counted as one instance of a backup. This statement is true even if there are multiple copies of the backup pieces that make up backup set, such as when a backup set has been backed up from disk to tape.

直译是：

一个备份集被算作一次备份， 这种算法甚至在多个备份片组成一个备份集时成立，  例如当备份集被从磁盘备份到磁带。



For the purposes of a recovery window retention policy, either all of the copies of a backup set are obsolete, or none of them are.


由于recovery window保留策略的缘故， 要么所有备份集合的拷贝都是 obsolete的，要么都不是


以上译文哪一点无法理解？ 请 逻辑推理指出！

xiangni330

谢谢两位解答！backup backupset的保留策略基本搞懂了。
但是我看到一道题，答案是B，我觉得应该是A（无论是恢复窗口还是冗余，对于备份集的备份，要么都obsolete要么都不obsolete），不知道是不是答案错了。
which statement describes the effect of a backup rentention plicy on the backup of a backup set?
A.Either all the copies of a backup set are obsolete or none of them are as per the rentetion policy.
B.All the copies of the backup set are counted as one instance of a backup and will be deleted if backup set exceeds the reundancy-based backup rentention policy.