Algorithms for Recovery and Isolation Exploiting Semantics: Difference between revisions

Revision as of 10:06, 14 April 2016

Revision as of 18:31, 20 April 2016

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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 P on the entry exists in the DPT table. If it doesn't, then we do not have to worry about redoing this entry since the data persists on the disk. If page P exists in the DPT table, then we see whether the Sequence Number in the DPT is smaller than the Sequence Number of the log record (i.e. whether the change in the log is newer than the last version that was persisted). If it isn't, then we don't redo the entry since the change is already there. If it is, we fetch the page from the database storage and check the Sequence Number stored on the page to the Sequence Number on the log record. If the former is smaller than the latter, the page does not needneeds to be written to the disk. That check is necessary because the recovered DPT is only a conservative superset of the pages that really need changes to be reapplied. Lastly, when all the above checks are finished and failed, 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 ===