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by issuing "alter tablespace begin backup;" what happens to
tablespace?
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Answer Posted / naveen
when u place a tablespace/database in backup (restore) mode(usually it's better idea to place only one tablespace at a time, as more logging is written, it should be done during a low activity period. And for the same reason, it is better to place the tablespaces one after one instead of putting all the tablespaces in backup mode)
when u alter the tablespace in begin backup mode - The datafile header is frozen so that whenever tablespace/datafile is copied, it reflects the checkpointSCN that was at the beginning of the backup. Then, when the copy will be restored, Oracle knows that it needs to start recovery at that SCN to apply the archived redo logs.This is to avoid the header inconsistency issue.That means that any further checkpoints do not update the datafile header SCN (but they do update a 'backup' SCN).
Each first modification to a block in buffer cache will write the full block into the redolog (in addition to the default behaviour that writes only the change vector).
At checkpoint the datafile header SCN is not updated - when updating a block, the first time it is updated since it came in the buffer cache,the whole before image of the block is recorded in redo- direct path writes do not go through the buffer cache, but they always write full blocks and then full block is written to redo log (if not in nologging)
This is to avoid the fractured block issue. There may be a fractured block in the copy, but it will be overwritten during the recovery with the full block image.
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