Note_Tech

All technological notes.

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DBA - Backup Rentention Policy

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• DBA - Backup Rentention Policy
  • Take-Away:
  • Retention Policy
    • Recovery Window Retention Policy
    • Redundancy Retention Policy
    • Disabling the Retention Policy
  • Batch Deletes of Obsolete Backups

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Take-Away:

Command	Description
SHOW RETENTION POLICY;	Show the current retention policy
CONFIGURE RETENTION POLICY CLEAR;	To revert to the default value
CONFIGURE RETENTION POLICY TO REDUNDANCY 1;	Default, Enable Redundancy Retention Policy
CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF <days> DAYS;	Enable Recovery Window Retention Policy
CONFIGURE RETENTION POLICY TO NONE;	Disable Retention Policy
REPORT OBSOLETE;	Report obsolete backup
DELETE OBSOLETE;	Delete obsolete backup

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Retention Policy

• retention policy

  • determines how long backups are kept for and how many copies are retained.
  • can set the value of the retention policy by using the RMAN CONFIGURE command or Enterprise Manager.

• There are two types of retention policies:

  • Recovery window: Establishes a period of time within which point-in-time recovery must be possible
  • Redundancy: Establishes a fixed number of backups that must be kept

• Retention policies are mutually exclusive.

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Recovery Window Retention Policy

• Recovery Window

  • a period of time during which it will be possible to discover logical errors and fix the affected objects by doing a point-in-time recovery to just before the error occurred.

• Recovery Window Retention Policy

  • specified in number of days.

  • If configure the RECOVERY WINDOW OF <days> DAYS, the window stretches

    • from the current time, which is the value of SYSDATE
    • to the point of recoverability, which is the earliest date to which you want to recover.
    • point of recoverability: SYSDATE - <days>
  • For each data file, there must always exist at least one backup that satisfies the following condition:
    • SYSDATE - backup_checkpoint_time >= recovery_window

• To prevent the record of older backups from being overwritten in the control file.

recovery catalog	CONTROL_FILE_RECORD_KEEP_TIME parameter
not using	>=recovery window
using	>=the time period between catalog resynchronizations

• Resynchronizations happen when:

  • Create a backup. In this case, the synchronization is done implicitly.
  • Execute the RESYNC CATALOG command.

• Command:

-- where <days> is the size of the recovery window.
CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF <days> DAYS;

• Disadvantage:
  • Although the recovery window is the best practice for specifying a retention policy, it can complicate disk space usage planning because the number of backups that must be kept by the recovery window is not constant and depends on the backup schedule. 原因在于恢复窗口期内备份的个数受到备份日程的影响, 从而造成磁盘空间难以计划.
  • 此时可以使用 Redundancy

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• Example:

  • a retention policy requiring recovery capability for any point within the last seven days.

• Configuring policy:

CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 7 DAYS;

• Q: for point-in-time recovery, which file is needed?

  • Some backups and logs are now obsolete because they are unnecessary for recovery within the seven-day window.
  • Based on the backups and archived log files available, only Backup B and logs 201 through 500 are needed to recover to any point within the recovery window.
    • Backup A is not required because there is a later backup (Backup B) that was taken before the recovery window.
    • Backup C alone does not suffice because it doesn’t allow recovery to points in time at the beginning of the recovery window.

• Take away

  • The needed backup for point-in-time recovery:
    • The most recent backup taken before the start of the recovery window, along with all subsequent logs, is necessary for complete recovery.
      • SYSDATE - backup_checkpoint_time >= recovery_window 恢复窗口外
      • the most recent one. 最近的备份

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Redundancy Retention Policy

• Redundancy-based retention policy

  • specifies how many backups of each datafile must be retained.
  • Used to maintain a specific number of backups.
  • requires cataloging a specified number of backups before any backup is considered obsolete.
    • A backup is considered obsolete when a newer version of the same file has been backed up.新版本
  • Default: 1

• Command:

-- Replace <copies> with the desired number of copies to meet your policy requirements.
CONFIGURE RETENTION POLICY TO REDUNDANCY <copies>;

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Disabling the Retention Policy

• Disabling the Retention Policy if:

  • using a separate system outside of RMAN to back up your disk backups to tape.

• Consequences of disabling the retention policy:

  • RMAN will never mark a backup as obsolete.
    • Since RMAN won’t need to decide when to remove a backup from disk (as another utility will manage that), it doesn’t need to be configured for this decision.
  • RMAN will retain records of each backup for the duration specified by the CONTROL_FILE_RECORD_KEEP_TIME initialization parameter.

• Exceptions:

  • You can make a backup an exception to the retention policy by creating an archival backup.

• Command:

CONFIGURE RETENTION POLICY TO NONE;

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Batch Deletes of Obsolete Backups

• Backup files can be marked as expired or obsolete. 备份文件标记

  • Expired:
    • RMAN has performed a crosscheck and the file cannot be found
  • Obsolete
    • based on the retention policy the file is not needed for recovery

• The backup retention policy only applies to

  • backup type:
    • full or level 0
  • File type:
    • datafile and control file.

• The retention policy does not directly affect archived redo logs and incremental level 1 backups.

  • These files become obsolete when no full backups exist that need them. 当其需要的父备份不存在时

• Datafile backup sets cannot be deleted until all datafile backups within the backup set are obsolete. 整体性

-- identify which backups are obsolete
REPORT OBSOLETE;

-- Deletes obsolete data file backups, copies, archived redo log files and log backups.
DELETE OBSOLETE;

• Best practice:
  • should run DELETE OBSOLETE periodically to minimize space wasted by storing obsolete backups.
    • e,g,.can run DELETE OBSOLETE in a weekly script.

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