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In the Oracle RDBMS environment, redo logs comprise files in a proprietary format which log a history of all changes made to the database. Each redo log file consists of redo records. A redo record, also called a redo entry, holds a group of change vectors, each of which describes or represents a change made to a single block in the database.
The first phase, Analysis, computes all the necessary information from the logfile. The Redo phase restores the database to the exact state at the crash, including all the changes of uncommitted transactions that were running at that point in time. The Undo phase then undoes all uncommitted changes, leaving the database in a consistent state.
The changes are first recorded in the log, which must be written to stable storage, before the changes are written to the database. [2] The main functionality of a write-ahead log can be summarized as: [3] Allow the page cache to buffer updates to disk-resident pages while ensuring durability semantics in the larger context of a database system.
LNS (log-write network-server) and ARCH (archiver) processes running on the primary database select archived redo logs and send them to the standby-database host, [7] where the RFS (remote file server) background process within the Oracle instance performs the task of receiving archived redo logs originating from the primary database and ...
In an Oracle database, a change vector consists of a single change to a single data block. A change vector is the smallest unit of change recorded in a redo log (in its turn, a redo log comprises files in a proprietary format which logs a history of all changes made to the database).
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