Carefully plan backup retention periods. Ensure enough backup media (tapes) are available and that old backups are expired in-time to make media available for new backups. Off-site vaulting is also highly recommended. Frequently test your ability to recover and document all possible scenarios. Most failed recoveries are a result of organizational errors and miscommunication.
Alert log file
The alert log file is a special trace file for a database. It chronologically records messages and errors about database operations, such as internal errors, block corruption errors, and information about dministration operations, including database recovery.
A cancel-based recovery is performed by entering CANCEL (instead of a log file name or AUTO) at the recovery prompt.
To perform a change-based recovery, you terminate the recovery at a specified system change number (SCN). This approach is often applied when you recover a database in a distributed environment.
Complete Oracle recovery, also known as complete media recovery, is applied to a database running in ARCHIVELOG mode. Completerecovery with archiving allows you to recover your database up to the time just prior to the occurrence of media failure.
A control file is a binary file containing the name and creation time of the database, the names and locations of a database's datafiles and redo log files. Every time an instance of the database is started, its control file is used to identify the datafile and redo log file that must be open for the database to run properly.
The Database Writer is responsible for writing modified blocks from the database buffer cache back to the database files.
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Incomplete recovery reconstructs the database as it was at a specified time before the media failure.
Instance failure means an abrupt end to an Oracle instance either due to a hardware or software problem. The system global area and background processes cease to function. Therefore, any data in the buffer that had not already been written to a datafile at the time of the instance failure will be lost. On the other hand, complete recovery recovers the damaged datafiles as well as all data that had been committed
prior to the moment of instance failure.
Log Writer (LGWR)
Log Writer writes Redo Entries into the online redo logs. After a log fills, a log switch occurs. A minimum of two online redo logs is required by LGWR. Each log switch causes a new log sequence number to be used and assigned to the current log file.
Media failure refers to the error that occurs when you try to write or read a file that is required to operate the database. It is also called disk failure because there is a physical problem reading or writing physical files on the disk. Mirrored online redo logs: A mirrored online redo log is also called a multiplexed online redo log. It contains copies of online redo log files physically located on a separate
disk. Any changes made to one member of the group are recorded in other members.
Mirror online redo logs
Make frequent backups and duplex the archived log files.
Operating system backup
An operating system (O/S) backup is made using an operating system command. Operating system backups can be written to disk or tape in any format that a specific operating system supports.
This recovery process allows several datafiles on different disks to be recovered at the same time. Parallel recovery is a very effective way to minimize recovery time.
The Program/Process Global Area (PGA) is a memory region that contains data and controls information for a single server process or a single background process. The information contained in the PGA could be sort data, session information, cursor state, or stack space.The PGA is allocated when a process is created and de-allocated when the process is terminated.
To recover means to make all the restored files current to the same point in time.
To restore means to bring back an original copy of a file from backup by issuing the operating system COPY command. Before recovery is performed, you need to restore the database.
The process of recovering datafiles and control files by applying the redo logs is also known as "rolling forward."
System change number
System change number is a clock value for the Oracle database that describes a committed version of the database. It functions as a timestamp that helps to ensure transaction consistency.
System global area
The Oracle database employs several memory structures that are contained with the System Global Area (SGA). The System Global Area contains three main in-memory structures. These memory structures may be tuned or modified by the DBA to improve performance. These memory structures include Shared Pool, Database Buffer Cache, and Redo Log Buffer.
Tablespace point-in-time recovery (TSPITR)
Tablespace point-in-time recovery (frequently abbreviated as TSPITR) refers to a recovery of all datafiles in a tablespace to a specific time.
To perform a time-based recovery, you must specify a point-in-time in the process. The recovery is complete when that specified point-in-time is reached.
Topoisomerases are enzymes that participate in the overwinding or underwinding of DNA. The winding problem of DNA arises due to the intertwined nature of its double-helical structure. During DNA replication and transcription, DNA becomes overwound ahead of a replication fork. If left unabated, this torsion would eventually stop the ability of DNA or RNA polymerases involved in these processes to continue down the DNA strand.