Answer / kittu

by issuing above statement it freezes tablespace it won't
allow single transaction to datafile. by this time redologs are generated & excessive redologs also

Answer / ravindra

When alter tablespace begin backup?

Ans:Oracle will write all changes to the datafile by rising
checkpoint and freezes the datafile header with latest SCN.
and backup will take upto latest SCN and transactions are
updating to datafiles also changes will be generate many
archivelogs because all changed transactions information
written to Redologs.

Answer / 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.

Answer / suman

alter tablespace begin backup; it means we can take the
backup of tablespace in hotbackup begin backup mode will be
start

Answer / kittu

by issuing above statement it freezes tablespace it won't
allow single transaction to datafile

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