Recovery in Oracle
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Oracle is a relational database management system. Its architecture is based on client/server paradigm. Basically Oracle has two parts: Oracle database and Oracle instance. The Oracle database is a logical collection of data (tables) that are manipulated and a set of system files that are used to support the desired functions. The Oracle instance, on the other hand, is a set of software modules that are responsible for providing access to the database and manage database state transition through a number of processes, such as the Process Monitor (PMON), the System Monitor (SMON), the Database Writer (DBWR), and the Log Writer (LGWR). PMON performs the following functions (a) it recovers a user process when it fails, (b) restarts dispatcher and server processes when they die, and (c) recovers a failed transaction (rolls it back). SMON performs the following functions: (a) performs instance recovery, and (b) releases temporary segments and merges contiguous free extents to make larger size free blocks. LGWR performs the following functions: (a) saves redo log buffers to log files, (b) manages the checkpoint process (the actual checkpointing is done by DBWR process), and (c) writes redo log data at (i) the commit point, or (ii) at every three seconds, or (iii) when the redo log buffer is one-third full. In this section the role of these processes are describes in more detail .
KeywordsData File Version Number Change Vector Media Recovery File Header
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