a) R/3 work process submits the request for an update.
b) Shadow process handles the update by locking in the database
c) It takes a backup of PSAPROLL/ PSAPUNDO to roll back to the original state if the update is cancelled or abrupt.
d) The record is fetched into DB Buffer for allowing selection.
e) A copy of the record is taken to log buffer for modification
f) the data/ record is modified in log buffer. This cannot be kept for long in the log buffer as the log buffer size is small.
LOGWRITE (LGWR) is used to write the committed data into Redo log files (Origlog A, Origlog B, Mirrorlog A, B) from log buffer.
This is the first place where a record is written to the disk to ensure that committed changes are not lost. To ensure the disk consistency log writer writes into Origredolog and Mirrorredolog files (Both resides on two different physical disks).
When the data is committed System Change Number is generated to ensure the faster commit mechanism. Log writer writes the commit into Redo log files but when the size of Redo log file is full size 50MB. It will perform Log Switch.
It is a process of switching between RedologA to RedologB and Mirror Logs A to B
i.e. when RedologA is full it is closed for writing and RedologB is opened for writing.
There will be only one process.
The log writer process writes data from the redo log buffers to the redo log files on the disk.
The writer is activated under the following conditions
1. When a transaction is committed
2. Every 3 Seconds
3. When the redo log buffer is 1/3rd full or 1MB of committed.
4. When DBWR signals the writing of redo records to disk.
The log writer must always be running for an instance.
(Note: If the buffer size is more no updates will happen. The DB WP waits till the buffer total memory gets utilized and then updates the DB)
SYSTEM/ SHARED GLOBAL AREA
It is a memory area which resides purely on memory and it is lost when the system is stopped.
i.e. SGA is lost the complete cache information is lost. It takes time to build SGA when system is started that is why the system is slow after a restart.
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