oracle 10gR2 rman備份優化加強

    在oracle 10gR1版本之前,用rman備份時會掃描曾經使用過的塊(包括做過truncate table等等未回收的塊),

導致rman備份集大小會比真正使用資料庫的容量更大和備份時間更多。

以下為metalink文件解釋：

 For releases up to and including 10GR1:

Each datafile is fully scanned. RMAN backs up every block that has ever been written to even if it is currently on the free-list so for example, if a table is truncated blocks used by that table are still included in the backup. Only blocks that have never been written to are omitted (NULL compression).  Hence:

physical database size determines INPUT workload
the number of dirty blocks determines backuppiece size
oversizing files for future growth is costly to RMAN as the whole file still has to be scanned with very little output

昨天有個客戶用9iR2 rman備份資料庫就發現這個問題，真正資料庫容量只有150G,而備份集大小就有200G,

導致規劃儲存容量用來做備份時產生不準確的估算。

10gR2版本就做了改進，備份優化，只備份真正使用過的塊，可以提高備份空間，效能和效率

metalink解釋：

Release 10GR2 and later:

If a tablespace is locally managed (LMT), compatible is set to 10.2 or later and a full or level 0 backup is being done to DISK, rman will only scan blocks that are CURRENTLY allocated to an object (Unused Block Compression).  So for example, if a table is truncated blocks used by that table will not be scanned by RMAN. Unused Block Compression results in improved backup performance by reducing the number of blocks scanned hence:

the space bitmap index for an LMT determines INPUT workload
the number of dirty blocks within those scanned determines backuppiece size
pre-allocating oversized extents to an object can be wasteful for RMAN as the whole extent will be scanned with relatively little output
Unused Block Compression cannot be used by:

3rd party media managers - the whole file is scanned every time a tape backup is done
Incremental backups – to get faster incremenals use Block Change Tracking
RMAN backup VALIDATE command
Oracle Secure Backup is the only media manager able to take advantage Unused Block Compression.

在給客戶做10gR2 rman備份時，備份集大小與資料庫容量大小非常相近。

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