Identify advantages and components of Fast-Start recovery.
Identify Components of Fast-Start recovery
Minimizing the failure or downtime of a database is an important goal for a systems administrator.
To help meet this goal, Oracle introduced (TAF) Transparent Application Failover. This feature, which works on Oracle Parallel Server, seamlessly migrates connections if any nodes fail, re-establishing the connections on other nodes.
This results in complete transparent migration of the users to the failover node, providing continuous availability of a database in case of database outages. This also allows the user to cache the queries on the client machine and re-instantiate them within the failover node.
Historical Note
Oracle8i introduced Fast-Start Fault Recovery to help systems administrators quickly recover the database from system faults and further minimize database downtime. Fast-Start Fault Recovery improves the recovery time when a database crashes, and consists of the following features:
Fast-Start checkpointing
Fast-Start on-demand rollback
Fast-Start parallel rollback
Checkpointing
One way to increase the performance of a database is through checkpointing, which decreases the number of I/O operations needed to perform instance recovery.
In Oracle8i, this is accomplished by the Fast-Start checkpointing component of the Fast-Start Fault Recovery feature. We will look at checkpointing in more detail later in this module.
Checkpointing: The process of writing all transactions into the redo log files.
On-demand rollback
Fast-Start Fault Recovery also provides an on-demand rollback feature to help minimize database failure. When a row is locked due to a dead
transaction and that row is required by another transaction, on-demand rollback recovers only the data block required by the present
transaction and leaves the rest of the data blocks to be recovered within the background. This improves the availability of data to users.
Parallel rollback
Recovery of a database includes two phases--rolling forward and rolling back. During the roll forward phase, all the transactions within the
redo log files are rolled forward or implemented to the database. For example, before the crash of a database, there were 1,500 transactions
including 500 inserts, 500 updates, and 500 deletes to the different tables that were within the memory and not implemented or written to the database. When the database crashes, all these transactions are within the redo log files and during the roll forward phase, all these
transactions are written to the database. During the roll back phase, all the uncommitted transactions are rolled back. These phases can
happen serially or in parallel. When the process is serial, only one server process is associated with it and it processes one transaction at
a time. If the process is in parallel, several server processes are assigned the task of processing them.
Within Oracle8, rolling forward was accomplished in parallel but rolling back was done serially.
Oracle8i allows for rolling back to be done in parallel. Parallel rollback uses a group of server processes and recovers transactions in parallel. The
database determines when it is faster to recover in parallel or serially ( serial rollback), thus minimizing database downtime. Move your mouse over the following diagram to learn more about the different components of Fast-Start Fault Recovery.
Roll forward phase: In this process, which happens during recovery, all transactions within the redo log files are applied to the database.
Rollback: This is a process, where the Oracle server replaces the old values for a record when a transaction is not committed.
Parallel rollback: Rolling back of data within multiple parallel processes.
Serial rollback: Rolling back of data within a single serial process.
All the data buffers have to be read during recovery
All the dead transactions have to be recovered during recovery
During recovery, the rollback phase is a serial process.
Fast-Start Checkpointing reduces the number of data buffers to be read, reducing the recovery time.
On-Demand Rollback reduces recovery time by recovering dead transactions as required by new transactions.
Recovery time is reduced by Parallel Rollback, as multiple server processes roll back the transactions.