The instance abnormally terminates because of a power outage. Which statement is true about redo
log files during instance recovery?
A.
Inactive and current redo log files are required to accomplish recovery
B.
Online and archived redo files are required to accomplish instance recovery
C.
All redo log entries after the last checkpoint are applied from redo log files to data files
D.
All redo log entries recorded in the current log file until the checkpoint position are applied to
data files
The most crucial structure for recovery operations is the redo log, which consists of two or more preallocated files that store all changes made to the database as they occur. Every instance of an Oracle Database has an associated redo log to protect the database in case of an instance failure
Redo log files that are required for instance recovery are called active redo log files.
Redo log files that are no longer required for instance recovery are called inactive redo log files.
Instance and crash recovery are the automatic application of redo log records to Oracle data blocks after a crash or system failure.
Periodically, Oracle records a checkpoint. A checkpoint is the highest system change number (SCN) such that all data blocks less than or equal to that SCN are known to be written out to the data files. If a failure occurs, then only the redo records containing changes at SCNs higher than the checkpoint need to be applied during recovery
PURPOSE OF CHECKPOINTS
Database blocks are temporarily stored in Database buffer cache. As blocks are read,
they are stored in DB buffer cache so that if any user accesses them later, they are available in memory and need not be read from the disk. When we update any row, the buffer in DB buffer cache corresponding to the block containing that row is updated in memory. Record of the change made is kept in redo log buffer . On commit, the changes we made are written to the disk thereby making them permanent. But where are those changes written? The changes are recorded in online redo log files by flushing the contents of redo log buffer to them. If the instance crashed right now, the DB buffer cache will be wiped out but on restarting the database, Oracle will apply the changes recorded in redo log files to the datafiles.
Checkpoint is a trigger.
Checkpoint is a synchronization event at a specific point in time which causes some / all dirty blocks to be written to disk thereby guaranteeing that blocks dirtied prior to that point in time get written.