Database Reference
In-Depth Information
use the ARCHIVE_LAG_TARGET initialization parameter to set a maximum amount of time (in seconds) between
log switches. a typical setting for this parameter is 1,800 seconds (30 minutes). a value of 0 (default) disables this
feature. this parameter is commonly used in oracle data Guard environments to force log switches after the specified
amount of time elapses.
Tip
You can also query the OPTIMAL_LOGFILE_SIZE column from the V$INSTANCE_RECOVERY view to determine if your
online redo log files have been sized correctly:
SQL> select optimal_logfile_size from v$instance_recovery;
This column reports the redo log file size (in megabytes) that is considered optimal, based on the initialization
parameter setting of FAST_START_MTTR_TARGET . Oracle recommends that you configure all online redo logs to
be at least the value of OPTIMAL_LOGFILE_SIZE . However, when sizing your online redo logs, you must take into
consideration information about your environment (such as the frequency of the switches).
Determining the Optimal Number of Redo Log Groups
Oracle requires at least two redo log groups in order to function. But, having just two groups sometimes isn't enough.
To understand why this is so, remember that every time a log switch occurs, it initiates a checkpoint. As part of a
checkpoint the database writer writes all modified (dirty) blocks from the SGA to the data files on disk. Also recall
that the online redo logs are written to in a round-robin fashion and that eventually the information in a given log is
overwritten. Before the log writer can begin to overwrite information in an online redo log, all modified blocks in the
SGA associated with the redo log must first be written to a data file. If not all modified blocks have been written to the
data files, you see this message in the alert.log file:
Thread 1 cannot allocate new log, sequence <sequence number>
Checkpoint not complete
Another way to explain this issue is that Oracle needs to store in the online redo logs any information that would
be required to perform a crash recovery. To help you visualize this, see Figure 2-2 .
 
 
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