Basic Concepts of Block Media Recovery

Basic Concepts of Block Media Recovery

Whenever block corruption have been automatically detected, you can perform block media recovery manually with the RECOVER ... BLOCK command. By default, RMAN first searches for good blocks in the real-time query physical standby database, then flashback logs and Then blocks with full or level 0 incremental backups.

Identification of corrupt Blocks

The V$database_block_corruption view displays blocks marked corrupt by DATABASE components such as RMAN, ANALYZE, DBV, and SQL queries.

The following types of corruption result in the addition of rows to this view:

Physical corruption (sometimes called media corruption) The database does not recognize the block:the checksum are invalid, the block contains all zeros, or the block header is C Orrupt. physical corruption checking is enabled by default. You can turn off checksum checking by specifying the nochecksum option for the BACKUP command, but other physical cons Istency checks, such as checks of the block headers and footers, cannot be disabled.

Logical corruption:

The block has a valid checksum, the header and footer match, and so on, but the contents is logically inconsistent. Block media recovery May is able to repair all logical block corruptions. In these cases, alternate recovery methods, such as tablespace point-in-time recovery, or dropping and re-creating the AF fected objects, may repair the corruption. Logical corruption checking is disabled by default. You can turn it on by specifying the CHECK LOGICAL option of the BACKUP, RESTORE, RECOVER, and VALIDATE commands.

The database can detect some corruptions by validating relationships between blocks and segments, but cannot detect them b Y a check of an individual block.

The V$database_block_corruption view does not record at this level of granularity.

Prerequisites for Block Media Recovery (link)

The following prerequisites apply to the RECOVER ... BLOCK command:

The target database must run in ARCHIVELOG mode and is open or mounted with a current control file.

If The target database is a standby database, then it must are in a consistent state, recovery cannot are in session, and th E backup must be older than the corrupted file.

The backups of the data files containing the corrupt blocks must is full or level 0 backups and not proxy copies.

If only proxy copy backups exist and then you can restore them to a nondefault location on disk, in which case RMAN considers them data file copies and searches them for blocks during block media recovery.

RMAN can use only archived redo logs for the recovery. RMAN cannot use level 1 incremental backups. Block Media Recovery cannot survive a missing or inaccessible archived redo log, although it can sometimes survive missing Redo Records.

Flashback database must is enabled on the target Database for RMAN to search the Flashback logs for good copies O F corrupt blocks. If Flashback logging is enabled and contains older, uncorrupted versions of the corrupt blocks, then RMAN can use these BL Ocks, possibly speeding up the recovery. The target database must is associated with a real-time query physical standby database for RMAN to search the database fo R good copies of corrupt blocks.

Purpose of Block Media Recovery

You can use block media recovery to recover one or more corrupt data blocks within a data file. Block Media Recovery provides the following advantages over data file Media recovery:

Lowers the mean time to recover (MTTR) because only blocks needing recovery is restored and recovered.

Enables affected data files to remain online during recovery

Without block media recovery, if even a single block was corrupt, then you must take the data file offline and restore a BA Ckup of the data file. You must apply any redo generated for the data file after the backup is created. The entire file is unavailable until media recovery completes. With block media recovery, only the blocks actually being recovered is unavailable during the recovery

Block Media Recovery is the most useful for physical corruption problems, involve a small, well-known number of blocks. Block-level data loss usually results from intermittent, random I/O errors that does not cause widespread data loss, and MEM Ory corruptions that is written to disk. Block Media Recovery is isn't intended for cases where the extent of data loss or corruption are unknown and the entire data File requires recovery. In such cases, data file Media recovery are the best solution.

Basic Concepts of Block Media Recovery