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14: Configuring the Database for Backup and Recovery
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A critical part of many service level agreements is the MTTR the mean time to recover after various events Instance recovery guarantees no corruption, but it may take a considerable time to do its roll forward before the database can be opened This time is dependent on two factors: how much redo has to be read, and how many read/write operations will be needed on the datafiles as the redo is applied Both these factors can be controlled by checkpoints A checkpoint guarantees that as of a particular time, all data changes made up to a particular SCN, or System Change Number, have been written to the datafiles by DBWn In the event of an instance crash, it is only necessary for SMON to replay the redo generated from the last checkpoint position All changes, committed or not, made before that position are already in the datafiles; so clearly, there is no need to use redo to reconstruct the transactions committed prior to that Also, all changes made by uncommitted transactions prior to that point are also in the datafiles so there is no need to reconstruct undo data prior to the checkpoint position either; it is already available in the undo segment on disk for the necessary rollback The more up to date the checkpoint position is, the faster the instance recovery If the checkpoint position is right up to date, no roll forward will be needed at all the instance can open immediately and go straight into the rollback phase But there is a heavy price to pay for this To advance the checkpoint position, DBWn must write changed blocks to disk Excessive disk I/O will cripple performance But on the other hand, if you let DBWn get too far behind, so that after a crash SMON has to process many gigabytes of redo and do billions of read/write operations on the datafiles, the MTTR following an instance failure can stretch into hours Tuning instance recovery time used to be largely a matter of experiment and guesswork It has always been easy to tell how long the recovery actually took just look at your alert log, and you will see the time when the STARTUP command was issued and the time that the startup completed, with information about how many blocks of redo were processed but until release 9i of the database it was almost impossible to calculate accurately in advance Release 9i introduced a new parameter, FAST_START_MTTR_TARGET, that makes controlling instance recovery time a trivial exercise You specify it in seconds, and Oracle will then ensure that DBWn writes out blocks at a rate sufficiently fast that if the instance crashes, the recovery will take no longer than that number of seconds So the smaller the setting, the harder DBWn will work in an attempt to minimize the gap between the checkpoint position and real time But note that it is only a target you can set it to an unrealistically low value, which is impossible to achieve no matter what DBWn does Database Control also provides an MTTR advisor, which will give you an idea of how long recovery would take if the instance failed This information can also be obtained from the view V$INSTANCE_RECOVERY
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The parameter FAST_START_MTTR_TARGET defaults to zero This has the effect of maximizing performance, with the possible cost of long instance recovery times after an instance failure The DBWn process will write as little as it can get away with, meaning
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that the checkpoint position may be a long way out of date and that therefore a large amount of redo would have to be applied to the datafiles in the roll forward phase of instance recovery Setting FAST_START_MTTR_TARGET to a nonzero value has two effects First, it sets a target for recovery, as described in the preceding section But there is also a secondary effect: enabling checkpoint auto-tuning The checkpoint auto-tuning mechanism inspects statistics on machine utilization, such as the rate of disk I/O and CPU usage, and if it appears that there is spare capacity, it will use this capacity to write out additional dirty buffers from the database buffer cache, thus pushing the checkpoint position forward The result is that even if the FAST_START_MTTR_TARGET parameter is set to a high value (the highest possible is 3600 seconds anything above that will be rounded down), actual recovery time may well be much less TIP Enabling checkpoint auto-tuning with a high target should result in your instance always having the fastest possible recovery time that is consistent with maximum performance Database Control has an interface to the parameter From the database home page, take the Advisor Central link, and then the MTTR advisor to get a window that displays the current estimated recovery time (this is the advisor) and gives the option of adjusting the parameter More complete information can be gained from querying the V$INSTANCE_RECOVERY view, described here:
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SQL> desc v$instance_recovery; Name Null ----------------------------------------- -------RECOVERY_ESTIMATED_IOS ACTUAL_REDO_BLKS TARGET_REDO_BLKS LOG_FILE_SIZE_REDO_BLKS LOG_CHKPT_TIMEOUT_REDO_BLKS LOG_CHKPT_INTERVAL_REDO_BLKS FAST_START_IO_TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR CKPT_BLOCK_WRITES OPTIMAL_LOGFILE_SIZE ESTD_CLUSTER_AVAILABLE_TIME WRITES_MTTR WRITES_LOGFILE_SIZE WRITES_LOG_CHECKPOINT_SETTINGS WRITES_OTHER_SETTINGS WRITES_AUTOTUNE WRITES_FULL_THREAD_CKPT Type -----------NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER
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The critical columns are described in Table 14-1 TIP Tracking the value of the ESTIMATED_MTTR will tell you if you are keeping to your service level agreement for crash recovery time; WRITES_ MTTR tells you the price you are paying for demanding a fast recovery time
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