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Journals

Teradata Database supports tables that are devoted to journaling. A journal is a record of some kind of activity. Teradata Database supports several kinds of journaling. The system does some journaling on its own, while you can specify whether to perform other journaling.

1. Down AMP recovery Occurs always

The Down AMP Recovery Journal (DARJ) is started on all AMPs in the cluster when an AMP is down. This allows for three AMPs to check on their mate. Since there are four AMPs in most clusters and all Fallback for a particular AMP remains within the cluster there are Three AMPs that will hold Fallback rows for a down AMP.

The Down AMP Recovery Journal (DARJ) is a special journal used only for FALLBACK rows when an AMP is not working. Like the TRANSIENT JOURNAL, the DARJ, also known as the RECOVERY JOURNAL, gets it space from the DBC’s PERM Space. When an AMP fails, the rest of the AMPs in its cluster initiate a DARJ. The DARJ keeps track of any changes that would have been written to the failed AMP. When the AMP comes back online, the DARJ will catch-up the AMP by completing missed transactions. Once everything is caught-up the DARJ is dropped.

• Is active during an AMP failure only
• Journals fallback tables only
• Is used to recover the AMP after the AMP is repaired, then is discarded

2. Transient Occurs always

A Transient journal maintains data integrity when in-flight transactions are interrupted. Data is returned to original state after the transaction failure.

A transient journal is used during normal system operation to keep"Before Images" of changed rows so that data can be restored to its previous state if the transaction is not completed. This happens on each AMP as changes occur. When a transaction is started, the sytem automatically stores all the rows affected by the transaction in the transient journal until the transaction is completed. Once the transaction is completed the "Before Images" are purged.

In the event of transaction failure, the "Before Images" are reapplied to the affected tables and deleted from the journal, and "rollback" operation is completed.


• Logs BEFORE images for transactions
• Is used by system to roll back failed transactions aborted either by the user or by the system
Captures:
• Begin/End Transaction indicators
• Before row images for UPDATE and DELETE statements
• Row IDs for INSERT statements
• Control records for CREATE, DROP, DELETE, and ALTER statements
• Keeps each image on the same AMP as the row it describes
• Discards images when the transaction or rollback completes

3. Permanent As specified by the user.

Permanent Journals are an optional feature used to provide an additional level of data protection. You specify the use of permanent journal at the table level. It provides full table recovery to a specific point in time. It can also reduce the need for costly and time - consuming full table backups.

Permanent journals are tables stored on disk array like user data is, so they can take up additional disk space, on the system. The database administrator maintains the permanent journal entries.

A database can have one permanent journal.

While creating a table with this option need to specify

1. Before Images
2. After Images

• Is available for tables or databases
• Can contain before images, which permit rollback, or after images, which permit rollforward, or both before and after images
• Provides rollforward recovery
• Provides rollback recovery
• Provides full recovery of nonfallback tables
• Reduces need for frequent, full-table archives

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