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MCITP SQL Server 2005 Database Administration All-in-One Exam Guide
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Next is checkpoint 3 At checkpoint 3 the transactions on lines 5, 6, and 7 would be committed Notice the transaction on line 8 is still open This may be referred to as a long-running transaction, though long-running transactions typically span many more than just three checkpoints A logical question is What causes a checkpoint to occur There s actually quite a long list, but be aware that checkpoints occur very frequently, often at least once per minute when the database is being modified Primarily, checkpoints are associated with the recovery interval option This option is an advanced server option and is set to 0 by default This allows the server to automatically adjust the interval, but generally, this means that the checkpoints occur as often as needed to ensure that recovery can occur within a minute upon reboot if a server fails During recovery, any committed transactions in the transaction log that haven t been written to the database are now written to the database Other events that cause checkpoints are the following:
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A database backup is about to start The transaction log becomes 70 percent full and it s in log-truncate mode SQL Server service is stopped
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Checkpoints and Recovery
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Checkpoints mark when data has been written to the database This is especially useful if we suffer a server failure and need to recover from it Consider Figure 11-6
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Figure 11-6 Using the transaction log to recover from a failure
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1 2 3 4 5 6 7 8
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INSERT Start Start Commit Commit UPDATE Start Commit Start Commit DELETE Start
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Checkpoint 0
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Checkpoint 1
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Checkpoint 2
System failure
Let s say that all the shown checkpoints have occurred Then our system experiences a failure for whatever reason, the server loses power and it goes down Notice that on line 8, we have a transaction that has started but not committed Additionally, note that on lines 6 and 7, we have transactions that have ended before a checkpoint these have not been written to disk
11: Data Recovery Strategies
Recovery Interval is a server option that can be set to indicate how often to do checkpoints Literally, the setting is indicating the maximum amount of time it would take to recover the database if a failure such as a power loss were to occur Indirectly, this setting tells the server how often to do checkpoints If we do checkpoints every 5 minutes, then if the server were to experience a power failure and the database files weren t corrupted, the transaction log wouldn t have more than 5 minutes of data to recover once power had returned This setting has always referred to minutes, though in practice, faster CPUs would perform more quickly In other words, a setting of 10 indicates 10 minutes, but a server would generally recover much more quickly than in 10 minutes It s rare to manipulate this setting The only reason to do so is if you notice that frequent checkpoints are adversely affecting performance
When SQL Server starts back up, it looks at the checkpoint log and the transaction log and determines it has work to do Lines 1, 2, 3, 4, and 5 were all committed before checkpoint 2 At checkpoint 2, they were written to the database The recovery process doesn t need to do anything with these lines However, on line 6, we have a transaction that started before checkpoint 2 and ended after checkpoint 2 It was not written to the database The recovery process recognizes that this transaction was committed, so it will roll it forward and write all the modifications to the database The same thing occurs for the DELETE on line 7; it is rolled forward and written to the database Line 8 is a little different Here, we have a Start transaction, without a Commit Remember, the definition of a transaction is that all events in the transaction have to complete as a whole or they all fail as a whole In this case, they haven t completed, so whatever was done after the start is rolled back
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