Oracle7 Server Concepts

Performing Recovery in Parallel

Recovery reapplies the changes generated by several concurrent processes, and therefore instance or media recovery can take longer than the time it took to initially generate the changes to a database. With serial recovery, a single process applies the changes in the redo log files sequentially. Using parallel recovery, several processes can simultaneously apply changes from the redo log files.

One form of parallel recovery can be performed by spawning several Server Manager sessions and issuing the RECOVER DATAFILE command on a different set of datafiles in each session. However, this method causes each Server Manager session to read the entire redo log file.

Instance and media recovery can be parallelized automatically by specifying an initialization parameter or command-line options to the RECOVER command. The Oracle Server can use one process to sequentially read the log files and dispatch redo information to several recovery processes to apply the changes from the log files to the datafiles. The recovery processes are started automatically by Oracle, so there is no need to use more than one session to perform recovery.

What Situations Benefit from Parallel Recovery

In general, parallel recovery is most effective at reducing recovery time when several datafiles on several different disks are being recovered concurrently. Crash recovery (recovery after instance failure) and media recovery of many datafiles on many different disk drives are good candidates for parallel recovery.

Additional Information: The performance improvement from parallel recovery is also dependent upon whether the operating system supports asynchronous I/O. If asynchronous I/O is not supported, parallel recovery can dramatically reduce recovery time. If asynchronous I/O is supported, the recovery time may only be slightly reduced by using parallel recovery. Consult your operating system documentation to determine whether asynchronous I/O is supported on your system.

Recovery Processes

In a typical parallel recovery situation, one process is responsible for reading and dispatching redo entries from the redo log files. This is the dedicated server process that begins the recovery session, typically a Server Manager session or an application designed to use the ALTER DATABASE RECOVER ... command. The server process reading the redo log files enlists two or more recovery processes to apply the changes from the redo entries to the datafiles. Figure 24 - 2 illustrates a typical parallel recovery session.

Figure 24 - 2. Typical Parallel Recovery Session

In most situations, one recovery session and one or two recovery processes per disk drive containing datafiles needing recovery is sufficient. Recovery is a disk-intensive activity as opposed to a CPU-intensive activity, and therefore the number of recovery processes needed is dependent entirely upon how many disk drives are involved in recovery. In general, a minimum of eight recovery processes is needed before parallel recovery can show improvement over a serial recovery.