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  Log archive and change accumulation

Log archive and change accumulation:

When the log is archived, it can be compressed therefore that it is convenient for media recovery. For disk objects log records are able to be sorted by cylinder then track then sector then time. Most likely all the records in the archived log belong to completed transactions. Therefore one only needs to keep redo records of committed (not aborted) transactions. Further merely the most recent redo record (new value) need be recorded. This compressed redo log is called as a change accumulation log. Because it is sorted by physical address media recover becomes a merge of the image dump of the object and its change accumulation tape.

FAST_MEDIA_RECOVERY: PROCEDURE (IMAGE, CHANGE_ACCUMULATION_LOG);
DO WHILE ( ! END_OF_FILE IMAGE);
READ IMAGE PAGE;
UPDATE WITH REDO RECORDS FROM CHANGE_ACCUMULATION_LOG;
WRITE IMAGE PAGE TO DISK;
END
END;

This is a entirely sequential process (sequential on input files as well as sequential on disk being recovered) and thus is limited only by the transfer rates of the devices.

The construction of the change accumulation file is able to be done off-line as an idle task.

If media errors are rare as well as availability of the data isn’t a critical problem then one may run the change accumulation utilities when needed. This may set aside building change accumulation files that are never used.

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