Three phases of recovery : the analysis phase, the redo phase, and finally the undo phase.

Question

SQL Server database recovery consists of three phases: the analysis phase, the redo phase, and finally the undo phase.

Can anybody please explain clearly or give a good link on these three phases of recovery??

Answer 1

From Understanding How Restore and Recovery of Backups Work in SQL Server

A restore is a multiphase process. The possible phases of a restore include the data copy, redo (roll forward), and undo (roll back) phases:

• The data copy phase involves copying all the data, log, and index pages from the backup media of a database to the database files.
• The redo phase applies the logged transactions to the data copied from
  the backup to roll forward that data
  to the recovery point. At this point, a database typically has uncommitted
  transactions and is in an unusable
  state. In that case, an undo phase is required as part of recovering the
  database.
• The undo phase, which is the first part of recovery, rolls back any
  uncommitted transactions and makes
  the database available to users.
  After the roll back phase, subsequent backups cannot be restored.