Oracle "hot" backups involve putting tablespace files into "backup" mode
while the data is copied.  You can either place the entire database in this
mode, or do this a tablespace file at a time, however you choose.  Note that
during this process, the tablespace copies will be "fuzzy" since Oracle
(from 7.3.2 up) updates them while you are backing them up, however, this is
OK, because the archive/redo logs do special logging of tablespaces that are
in backup mode and are aware of when they are placed in this mode and when
they come out.  The database is fully available for use, the logs are the
key to consistent recovery.  I'm not aware of actual locking being done in
either case (table or row level).

This is based on experience with Oracle database backups using user
developed scripting and BACKINT with SAP.

Thanks,
Bernie Survoy
MVS Systems Engineer
216.615.9324 phone
216.621.5920 fax
StorageTek
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