Undo Pass

• 3rd pass of the log during restart recovery.
• A routine (RESTART_UNDO) is written that implements the undo pass actions.
• The input to this routine is the restart transaction table.
• The dirty_pages table is not consulted during the undo pass.
• Since history is repeated before the undo pass is initiated, the LSN on the page is not consulted to determine whether an undo operation should be performed or not.
• The RESTART_UNDO routine rolls back loser transactions, in reverse chronological order, in a single sweep of the log.
• This is done by continually taking the maximum of the LSNs of the next log record to be processed for each of the yet-to-be-completely-undone loser transactions, until no loser transaction remains to be undone.
• The next log record to process for each transaction to be rolled back is determined by an entry in the transaction table for each of those transactions.
• In the process of rolling back of transactions, CLRs are written.
• The buffer manager follows the usual write ahead log (WAL) protocol while writing dirty pages to nonvolatile storage during the undo pass.
• With ARIES, we have the option of allowing the continuation of loser transactions after restart recovery is completed.
• Because of repetition of history and savepoint concepts, in the undo pass, we could roll back each loser transaction only to its latest savepoint, instead of total rollback.