Database Reference
In-Depth Information
There are two different kinds of Oracle recoveries: instance/crash recovery
and media recovery.
Instance recovery
is automatically managed by Oracle
when you restart the database.
Media recovery
is a manual process done
by the DBA and involves the use of Oracle Database backups.
Oracle Processes Related to Backup and Recovery
The front-line support for Oracle backup and recovery is the Oracle architecture. One part
of this architecture is the processes related to the Oracle Database. Although the Oracle
Database has a number of processes, only a few really matter with respect to backup and
recovery and will be mentioned in this text. These processes are as follows:
■
LGWR
■
DBW
■
ARCH
■
Multithreaded Oracle
■
User processes
Let's discuss each of these processes next so you can better understand how they impact
database recovery.
LGWR Process
The
log writer process (LGWR)
is responsible for keeping the online redo logs up to date.
The job of the LGWR process is to move redo from the volatile (nonpersistent) redo log
buffer in the System (sometimes called Shared) Global Area (SGA) to the persistence of the
online redo logs. A number of different things cause LGWR to wake up and write the redo
data, among them are database session commit operations and when the redo log buffer
fills to a certain point.
DBW Processes
The
database writer processes (DBWn)
(the
n
indicates that there might be more than one of
these processes) are responsible for writing to the database data files. This writing occurs dur-
ing events called
checkpoints
. A database checkpoint may, in reality, happen at just about any
time while the database is running. DBW has very little to do with recovery of the database
(other than to support it by writing to the data files) because database data file writes are often
delayed and the blocks within the data files themselves are not consistent with the current state
of the data inside of the SGA. By default, Oracle starts only one DBW process, but if needed,
additional DBW processes can be utilized, though this is rare.
ARCH Processes
The
archiver processes (ARCn)
are responsible for the creation of archived redo logs. In a later
section in this chapter, we will discuss how redo logs are filled with redo. Once the redo log file
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