Determining the time to recover from an instance failure is
a necessary component for reaching required service levelsagreements. For example, if service levels dictate that when a
node fails, instance recovery time can be no more than 3 minutes,
FAST_START_MTTR_TARGET should be set to 180
Fast-start checkpointing refers to the periodic writes by the database writer (DBWn)
processes for the purpose of writing changed data blocks from the Oracle buffer cache
to disk and advancing the thread-checkpoint. Setting the database parameter
FAST_START_MTTR_TARGET to a value greater than zero enables the fast-start
Fast-start checkpointing should always be enabled for the following reasons:
It reduces the time required for cache recovery, and makes instance recovery
time-bounded and predictable. This is accomplished by limiting the number of dirty
buffers (data blocks which have changes in memory that still need to be written to
disk) and the number of redo records (changes in the database) generated between
the most recent redo record and the last checkpoint.
Fast-Start checkpointing eliminates bulk writes and corresponding I/O spikes
that occure traditionally with interval- based
checkpoints, providing a smoother, more consistent I/O pattern that is more
predictable and easier to manage.
If the system is not already near or at its maximum I/O capacity, fast-start
checkpointing will have a negligible impact on performance. Although fast-start
checkpointing results in increased write activity, there is little reduction in
database throughout, provided the system has sufficient I/O capacity.
Check-pointing is an important Oracle activity which records the highest system
change number (SCN,) so that all data blocks less than or equal to the SCN are known
to be written out to the data files. If there is a failure and then subsequent cache
recovery, only the redo records containing changes at SCN(s) higher than the
checkpoint need to be applied during recovery.
As we are aware, instance and crash recovery occur in two steps - cache recovery
followed by transaction recovery. During the cache recovery phase, also known as the
rolling forward stage, Oracle applies all committed and uncommitted changes in the
redo log files to the affected data blocks. The work required for cache recovery
processing is proportional to the rate of change to the database and the time between
Mean time to recover (MTTR)
Fast-start recovery can greatly reduce the mean time to recover (MTTR), with minimal
effects on online application performance. Oracle continuously estimates the recovery
time and automatically adjusts the check-pointing rate to meet the target recovery
With 10g, the Oracle database can now self-tune check-pointing to achieve good
recovery times with low impact on normal throughput. You no longer have to set any
This method reduces the time required for cache recovery and makes the recovery
bounded and predictable by limiting the number of dirty buffers and the number of
redo records generated between the most recent redo record and the last checkpoint.
Administrators specify a target (bounded) time to complete the cache recovery phase
of recovery with the FAST_START_MTTR_TARGET initialization parameter, and
Oracle automatically varies the incremental checkpoint writes to meet that target.
The TARGET_MTTR field of V$INSTANCE_RECOVERY contains the MTTR target in effect. The
ESTIMATED_MTTR field of V$INSTANCE_RECOVERY contains the estimated MTTR should a
crash happen right away.
Enable MTTR advisory
Enabling MTTR Advisory Enabling MTTR Advisory involves setting two parameters:
STATISTICS_LEVEL = TYPICAL
FAST_START_MTTR_TARGET > 0
Estimate the value for FAST_START_MTTR_TARGET as follows:
TARGET_MTTR ESTIMATED_MTTR CKPT_BLOCK_WRITES
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FAST_START_MTTR_TARGET = 214;
Whenever you set FAST_START_MTTR_TARGET to a nonzero value, then set the following parameters to 0.
LOG_CHECKPOINT_TIMEOUT = 0
LOG_CHECKPOINT_INTERVAL = 0
FAST_START_IO_TARGET = 0
Disable MTTR advisory
FAST_START_MTTR_TARGET = 0
LOG_CHECKPOINT_INTERVAL = 200000