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The other type of recovery, restore recovery (or media recovery), is run by request when a restore operation is executed. This process makes sure that all the committed transactions in the backup of the transaction log are re ected in the data and that any transactions that did not complete do not show up in the data. I ll talk more about restore recovery later in the chapter. Both types of recovery must deal with two situations: when transactions are recorded as committed in the log but not yet written to the data les, and when changes to the data les don t correspond to committed transactions. These two situations can occur because committed log records are written to the log les on disk every time a transaction commits. Changed data pages are written to the data les on disk completely asynchronously, every time a checkpoint occurs in a database. As I mentioned in 1, SQL Server 2008 Architecture and Con guration, data pages can also be written to disk at other times, but the regularly occurring checkpoint operations give SQL Server a point at which all changed (or dirty) pages are known to have been written to disk. Checkpoint operations also write log records from transactions in progress to disk because the cached log records are also considered to be dirty. If the SQL Server service stops after a transaction commits but before the data is written out to the data pages, when SQL Server starts and runs recovery, the transaction must be rolled forward. SQL Server essentially redoes the transaction by reapplying the changes indicated in the transaction log. All the transactions that need to be redone are processed rst (even though some of them might need to be undone later during the next phase). This is called the redo phase of recovery. If a checkpoint occurs before a transaction is committed, it writes the uncommitted changes out to disk. If the SQL Server service then stops before the commit occurs, the recovery process nds the changes for the uncommitted transactions in the data les, and it has to roll back the transaction by undoing the changes re ected in the transaction log. Rolling back all the incomplete transactions is called the undo phase of recovery.
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Note I ll continue to refer to recovery as a system startup function, which is its most common
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role by far. However, remember that recovery is also run during the nal step of restoring a database from backup or attaching a database, and can also be forced manually. In addition, recovery is run when creating a database snapshot, during database mirroring, or when failing over to a database mirror.
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Later in this chapter, I ll cover some special issues related to recovery during a database restore. These include the three recovery models that you can set using the ALTER DATABASE statement and the ability to place a named marker in the log to indicate a speci c point to recover to. The discussion that follows deals with recovery in general, whether it s performed when the SQL Server service restarts or when a database is being restored from a backup.
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During recovery, only changes that occurred or were in progress since the last checkpoint are evaluated to determine if they need to be redone or undone. Any transactions that completed prior to the last checkpoint, either by being committed or rolled back, are accurately re ected in the data pages, and no additional work needs to be done for them during recovery. The recovery algorithm has three phases, which center around the last checkpoint record in the transaction log. The three phases are illustrated in Figure 4-1. Phase 1: Analysis The rst phase is a forward pass starting at the last checkpoint record in the transaction log. This pass determines and constructs a dirty page table (DPT) consisting of pages that might have been dirty at the time SQL Server stopped. An active transaction table is also built that consists of uncommitted transactions at the time SQL Server stops. Phase 2: Redo This phase returns the database to the state it was in at the time the SQL Server service stopped. The starting point for this forward pass is the start of the oldest uncommitted transaction. The minimum LSN in the DPT is the rst time SQL Server expects to have to redo an operation on a page, but it needs to redo the logged operations starting all the way back at the start of the oldest open transaction so that the necessary locks can be acquired. (Prior to SQL Server 2005, it was just allocation locks that needed to be reacquired. In SQL 2005 and later, all locks for those open transactions need to be reacquired.) Phase 3: Undo This phase uses the list of active transactions (uncommitted at the time SQL Server came down) which were found in Phase 1 (Analysis). It rolls each of these active transactions back individually. SQL Server follows the links between entries in the transaction log for each transaction. Any transaction that was not committed at the time SQL Server stopped is undone so that none of the changes are actually re ected in the database.
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