Fast-Start Parallel Rollback

While "Fast-Start" is a key part of Oracle's parallel rollback process, it's not a feature you directly "implement" yourself. Oracle automatically uses Fast-Start when necessary during instance recovery to expedite rollback of uncommitted transactions. Here's a breakdown of parallel rollback and the role of Fast-Start:
Parallel Rollback in Oracle:

• Purpose: When an Oracle instance recovers from a crash, it needs to undo uncommitted transactions. Parallel rollback speeds up this process by using multiple server processes to concurrently roll back transactions.
• Mechanism: The SMON background process acts as the coordinator, identifying uncommitted transactions and assigning them to available server processes. Each server process then rolls back its assigned transactions in parallel.

Fast-Start Feature:

• Role in Rollback: Fast-Start enables efficient identification of uncommitted transactions. Instead of scanning the entire undo segment sequentially, it uses dedicated data structures for faster transaction identification, significantly reducing the initial overhead of parallel rollback.
• Automatic Activation: You don't need to specifically configure Fast-Start. It's enabled by default in Oracle 19c and automatically invoked during instance recovery when parallel rollback is triggered.

Key Points:

• You don't directly "implement" Fast-Start, it's an internal mechanism used by Oracle for parallel rollback.
• Parallel rollback automatically activates when necessary during instance recovery.
• You can monitor and adjust parallel rollback settings like the degree of parallelism (number of server processes) using database parameters.

While you can't directly use Fast-Start independently, understanding its role in parallel rollback can help you optimize your instance recovery process by adjusting relevant parameters like the degree of parallelism.

More processes mean better performance. This is the main theme behind "Fast-Start parallel rollback". Oracle background processes coordinate and roll back a set of transactions by using multiple server processes. Whenever a large number of transactions (like INSERT, UPDATE, and DELETE statements) have not been committed, Fast-Start parallel rollback becomes very important.
When the System Monitor process realizes that a large number of transactions must be rolled back, and it is faster to roll back in parallel than serially, the System Monitor starts multiple server processes, rolling back the transactions. Each server process handles one transaction and the performance is improved.

Fast-start parallel rollback has a special feature called intra-transaction recovery^[1], whereby a single transaction is divided among several processes. This happens when one transaction is large, meaning it takes longer for one process to roll this transaction back than for the other processes to roll back their transactions. Oracle automatically begins intra-transaction recovery by dispersing the large transaction among the processes. Setting the parameter FAST_START_PARALLEL_ROLLBACK to one of the following three values controls the number of processes involved in transaction recovery:

1. FALSE: Turns off Fast-Start parallel rollback
2. LOW: Specifies that the number of recovery servers may not exceed twice the value of the CPU_COUNT parameter
3. HIGH: Specifies that the number of recovery servers may not exceed four times the value of the CPU_COUNT: parameter

An instance is recovered using RMAN, by issuing the RECOVER command. This is called “Fast-Start-parallel rollback,” and you must define the degree of parallelism while issuing the command.

The following illustration is an example of an initialization parameter file (INIT.ORA). Based on this information you will need to determine the parameter setting for Fast-Start-parallel rollback: false, low, or high. Click the View Image button to see the illustration.
The value assigned is LOW. By using this value, the number of processes created to perform parallel rollback will be twice the number of CPU’s within the host machine containing the database. If there are 4 CPU's within the host machine, the number of processes that will be created is 8.
Afer adding the parameter within INIT.ORA, shut down and restart the database for the parameters to take effect. Implementing this parameter within the database instance ensures faster recovery of non-committed transactions.

FAST_START_PARALLEL_ROLLBACK specifies the degree of parallelism used when recovering terminated transactions. Terminated transactions are transactions that are active before a system failure. If a system fails when there are uncommitted parallel
DML or DDL transactions, then you can speed up transaction recovery during startup by using this parameter. Values:

1. FALSE- Parallel rollback is disabled
2. LOW- Limits the maximum degree of parallelism to 2 * CPU_COUNT
3. HIGH - Limits the maximum degree of parallelism to 4 * CPU_COUNT

If you change the value of this parameter, then transaction recovery will be stopped and restarted with the new implied degree of parallelisme.
The next lesson explains how to set up an automated standby database.

[1]intra-transaction recovery: Intra-transaction recovery is a feature of Oracle backups that allows you to recover a database to a consistent point in time, even if a transaction was in progress when the backup was started. This is done by using a combination of redo logs and database snapshots.