Question: What is the
Data Guard Archive Gap Sequence? How do I get gaps in the
sequenece numbers of my archived redo logs?
Answer: A Data Guard Archive Gap Sequence
is a set of archived redo logs that could not be transmitted to the
standby site. As a result of an archive gap, the standby database
will lag behind the primary database.
Usually, a Data Guard archive gap sequence is created as the
result of a network outage and the connection between the primary
and the standby site is lost. The archive gap sequence can also
occur during the creation of a standby database.
Oracle
Corporation has significantly improved automatic archive gap
detection and resolution which reduces the chances of lag between
the primary and the standby site.
Data Guard Archive log gap Detection and Resolution
Three
methods of automatic archive gap detection became available.
The following is a description of their features:
- The archiver
process of the primary database polls the standby databases every 60
seconds. This is referred to as ?heartbeat.? During heartbeat, if a
standby database reports an archive gap, the archiver of the primary
database sends the archived redo log files required to fill the gap.
Once the files have been transferred, the site is marked as up to
date.
- Archived redo logs are transferred from the primary
database to the standby database. On the primary database side, the
LGWR or ARCH is responsible for sending the archived redo logs. On
the standby site, the RFS process takes the archived redo log file
and writes to the host machine. An archived redo log file is
uniquely identified by its sequence number and thread number.
- When the RFS process receives an archived redo log file, it
compares the sequence number with the sequence number of the
previously received archived redo log file. If the sequence number
of the current archived redo log file is greater than the sequence
number of the last received archived redo log file plus one, a
request is sent to the archiver process of the primary database that
is providing a list of missing archived redo log files. The archiver
of the primary database then retransmits these archived redo log
files, thereby resolving the gap sequence.
- For a physical
standby database, Oracle introduced a request-response system for
gap resolution through the FAL background process. The FAL method of
gap resolution is initiated by the Managed Recovery Process (MRP).
If the MRP detects a gap in archived redo logs during the
application of log files, it requests the archiver process of the
primary database to resend the archived redo log files which have
caused the gap sequence.
- There are two important
parameters required for gap resolution through the FAL background
process: fal_server and fal_client. These two parameters need to be
set in the initialization parameter file of the physical standby
database. fal_server is the service name of the primary database and
fal_client is the service name of the standby database.
-
The fal_client should be one of the remote archival destinations of
the primary database pointing to this standby database. When MRP
detects a gap sequence, it requests the archiver process of the
primary database, identified by the fal_server parameter, and passes
the sequence number of log files causing the archive gap. In
addition, it passes the service name defined by the fal_client
parameter to the archiver of the primary database. The archiver
process of the primary database then retransmits the archived log
files requested by the MRP of the standby database.
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