Algorithms for Recovery and Isolation Exploiting Semantics: Difference between revisions

Revision as of 05:34, 12 December 2013

Revision as of 05:20, 6 March 2014

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From the DPT we can compute the minimal Sequence Number of a dirty page. From there we have to start redoing the actions until the crash, in case they weren't persisted already.

Running through the log file we check for each entry whether the modified page is in the DPT table and whether the Sequence Number in the DPT is smaller than the Sequence Number of the record (i.e. whether the change in the log is newer than the last version that was persisted). If it is, we fetch the page from the database storage and check the Sequence Number on the actual if it is smaller than the Sequence Number on the log record. That check is necessary because the recovered DPT is only a conservative superset of the pages that really need changes to be reapplied. Lastly we reapply the redo action and store the new Sequence Number on the page. It is also important for recovery from a crash during the Redo phase, as the redo isn't applied twice to the same page.

=== Undo ===