Checkpoint=checkpoint-interval
Specifies, in terms of processed messages, how frequently the
Hot Standby servers update information in the database root file.
This qualifier can be set to different values on the master and
standby databases.
Applicable to: Master and standby database
Required or Optional: Optional
Default Value: 100 messages
Minimum Value: 1 message
Maximum Value: 1024 messages
By default, the Hot Standby servers automatically perform
checkpoint operations on both the master and standby databases
after every 100 messages are processed. Checkpoints are essential
to database availability because they:
o Enable the Hot Standby software to restart database
replication operations more quickly in the event of a failure
because frequent checkpoints limit the number of transactions
that must be redone if a process or system fails.
o Cause all modified database cache buffers on the standby
database to be flushed to the disk, making the buffers
available for access by other users (when online database
access is enabled).
o Improve the redo performance of the database recovery (DBR)
process.
o Allow after-image backup operations to back up older after-
image journals on the master database.
NOTE
In addition to performing checkpoint operations specified
by the Checkpoint qualifier, the replication servers on
the master database also checkpoint automatically after
the following events:
o After two minutes of inactivity
o After a switchover to a new after-image journal (when
you are using circular after-image journals)
o After an AIJ backup operation (when you are using
extensible after-image journals)
On the standby database, the LRS process checkpoints
after two minutes of inactivity if data has been
processed since the last checkpoint.
These automatic checkpoints advance the oldest active
checkpoint indicator to make older after-image journals
available for backup operations. You cannot change or
override these checkpoint intervals.
The default checkpoint interval usually is sufficient to
effectively maintain synchronization between the master and
standby database root files. However, you can override the
default checkpoint interval by specifying the Checkpoint
qualifier when you start replication on the master database,
the standby database, or both.
For example, if you specify the qualifier Checkpoint=300 on the
standby database, the LRS server process updates information
in the standby database root file after every 300 messages
are exchanged between the master and the standby database. The
following table describes how the frequency of the checkpoint
operation can affect database synchronization.
Table 24 Setting the Frequency of Checkpoint Intervals
If you specify . . . Then . . .
A small checkpoint The Hot Standby software synchronizes the
interval database root files more often, but uses
less time to restart replication because
fewer transactions need to be redone.
A large checkpoint The Hot Standby software synchronizes the
interval database root files less frequently, but
requires more time to restart replication
because more transactions must be redone.
In addition, the value you set for the checkpoint interval:
o Controls replication restart in the event of a failure on the
master database. A side effect of this is that the ABS process
cannot back up after-image journals that are needed to restart
replication operations.
o Affects how the after-image journals on the master database
become available for backup.
Specifying a large value for the checkpoint interval can
cause after-image journal backup operations to stall until
the appropriate after-image journal file becomes available for
a backup operation. This is because the after-image journal
backup operation cannot back up any after-image journal file
that is required for process recovery or replication restart.
o Affects the reinitialization of after-image journals on the
standby database.
o Affects the manner in which the LRS process on the standby
database:
- Releases page locks
- Responds to page lock conflict messages from another
attached database process
Oracle Corporation recommends that you set a reasonably small
checkpoint interval for the standby database. Specifying a
checkpoint interval that is too large can prevent the LRS
process from responding to requests for pages, and it is
possible for other processes to become stalled.
For Oracle Rdb databases, you can monitor the effectiveness of
the current setting of the Checkpoint qualifier by using the RMU
Show Statistics command and examining the Checkpoint Information
display.