How Transaction Information Aids Recovery

The SAP Sybase IQ recovery mechanism is designed for a data warehouse enivronment, in which, typically, transactions are few but lengthy.

SAP Sybase IQ performs database updates on a copy of the actual database page, and then writes the data to disk whenever a write transaction commits. SAP Sybase IQ also records:

The location and quantity of changed data for each transaction is stored in a transaction log.
The location of any version pages and free space is stored on disk. SAP Sybase IQ uses this information to free up space when versions are no longer needed. All versions created during a write transaction become obsolete when the write transaction commits or rolls back. Individual versions can be released at a savepoint.
Additional information about checkpoints that occurred during a transaction.

When you need to recover your database, SAP Sybase IQ restores quickly from the information in the transaction log and the checkpoint information. It uses the information about versions and free space to roll back transactions, and to release the disk space occupied by obsolete versions.

The transaction log requires very little space: only about 128 bytes for each committed transaction. The space requirements for checkpoint and disk space availability information are also very small. However, in systems with a high number of transactions that change data, the transaction log can grow to be very large, requiring periodic truncation.

The checkpoint information is deleted at the next checkpoint. Information related to particular savepoints is deleted when the savepoint is released or rolled back.