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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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that records the LSN of the log that needs to be undone next, when the transaction is being rolled back This eld serves the same purpose as the operation identi er in the operation-abort log record in our scheme, which helps to skip over log records that have already been rolled back The DirtyPageTable contains a list of pages that have been updated in the database buffer For each page, it stores the PageLSN and a eld called the RecLSN which helps identify log records that have been applied already to the version of the page on disk When a page is inserted into the DirtyPageTable (when it is rst modi ed in the buffer pool) the value of RecLSN is set to the current end of log Whenever the page is ushed to disk, the page is removed from the DirtyPageTable A checkpoint log record contains the DirtyPageTable and a list of active transactions For each transaction, the checkpoint log record also notes LastLSN, the LSN of the last log record written by the transaction A xed position on disk also notes the LSN of the last (complete) checkpoint log record
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Analysis pass: This pass determines which transactions to undo, which pages were dirty at the time of the crash, and the LSN from which the redo pass should start Redo pass: This pass starts from a position determined during analysis, and performs a redo, repeating history, to bring the database to a state it was in before the crash Undo pass: This pass rolls back all transactions that were incomplete at the time of crash Analysis Pass: The analysis pass nds the last complete checkpoint log record, and reads in the DirtyPageTable from this record It then sets RedoLSN to the minimum of the RecLSNs of the pages in the DirtyPageTable If there are no dirty pages, it sets RedoLSN to the LSN of the checkpoint log record The redo pass starts its scan of the log from RedoLSN All the log records earlier than this point have already been applied to the database pages on disk The analysis pass initially sets the list of transactions to be undone, undo-list, to the list of transactions in the checkpoint log record The analysis pass also reads from the checkpoint log record the LSNs of the last log record for each transaction in undo-list The analysis pass continues scanning forward from the checkpoint Whenever it nds a log record for a transaction not in the undo-list, it adds the transaction to undo-list Whenever it nds a transaction end log record, it deletes the transaction from undo-list All transactions left in undo-list at the end of analysis have to be rolled back later, in the undo pass The analysis pass also keeps track of the last record of each transaction in undo-list, which is used in the undo pass
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
V Transaction Management
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The McGraw Hill Companies, 2001
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The analysis pass also updates DirtyPageTable whenever it nds a log record for an update on a page If the page is not in DirtyPageTable, the analysis pass adds it to DirtyPageTable, and sets the RecLSN of the page to the LSN of the log record Redo Pass: The redo pass repeats history by replaying every action that is not already re ected in the page on disk The redo pass scans the log forward from RedoLSN Whenever it nds an update log record, it takes this action: 1 If the page is not in DirtyPageTable or the LSN of the update log record is less than the RecLSN of the page in DirtyPageTable, then the redo pass skips the log record 2 Otherwise the redo pass fetches the page from disk, and if the PageLSN is less than the LSN of the log record, it redoes the log record Note that if either of the tests is negative, then the effects of the log record have already appeared on the page If the rst test is negative, it is not even necessary to fetch the page from disk Undo Pass and Transaction Rollback: The undo pass is relatively straightforward It performs a backward scan of the log, undoing all transactions in undo-list If a CLR is found, it uses the UndoNextLSN eld to skip log records that have already been rolled back Otherwise, it uses the PrevLSN eld of the log record to nd the next log record to be undone Whenever an update log record is used to perform an undo (whether for transaction rollback during normal processing, or during the restart undo pass), the undo pass generates a CLR containing the undo action performed (which must be physiological) It sets the UndoNextLSN of the CLR to the PrevLSN value of the update log record
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