A physical standby
database is an exact, block-for-block copy of a primary database, maintained
through “Redo Apply” process. In this process, the redo data which is received
from the primary database is continuously applied to the physical standby
database using the database recovery mechanisms.
Background processes
involved:
On Primary
database:
®
LGWR:
- Flushes
log buffers from SGA to Online Redo log files.
- To
transmit redo data to Standby locations for standby databases configured with “Maximum
Availability” and “Maximum Protection” level of data protection mode.
®
ARCH:
- This
process on the Standby site generates archive log files from Onilne redo log
files.
- Can
have up to 30 ARCn processes in the databases. This can be done by setting
the LOG_ARCHIVE_MAX_PROCESSES initialization parameter (the
default setting is 4, but the maximum value is 30)
- This
archival process supports only for “Maximum Performance” level of data
protection mode.
®
LNS:
- LNS
process reads the redo flushed from the redo buffers by LGWR and transmits the redo
over the network to RFS on Standby database. By this, LNS process frees up the
LGWR process from performing the redo transporting role.
On Standby
database:
®
RFS:
- Receives
the redo data transmitted over the network from Primary site and then writes
the received network buffers to the Standby redo log files (SRL).
®
ARCH:
- This
process on the Standby site generates archive log files from Standby redo log
files.
®
MRP:
- Coordinates
media recovery management, that is, it applies the redo data in the generated archive
log files on the Standby database.
Advantages:
Ø
A physical standby database can be opened
for read-only access and used to offload queries from a primary database like
backups and reporting.
Ø
Real-time query feature can be used if the
license for the Oracle Active Data Guard option has been purchased.
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