Re: Hot Backups

Lee Parsons (lparsons_at_eskimo.com) wrote:
: In article <D78GMr.1yL_at_lanier.com>, Don Vick <dvick_at_lanier.com> wrote:
: >When a tablespace is in backup status, Oracle saves all
: >update activity in the redo logs and catches up the data files after the
: >backup is done. Redo logs cannot be archived if they contain active data,
: >so you run the risk of running out of online redo logs and stalling your
: >system (i.e., blocking udpates). (I may have some details wrong here, but
: >that is the general idea. :-)
 

: Not true. The system starts logging more information than usual but
: it doesn't have to keep it online. Changes are written to the datafiles
: just as if the system was not being backed up. The backups of this data
: is inconsistant therefor the offline archive files are needed to roll
: the datafile forward to a consistant state. But the disk version was
: never effected. With the exception of checkpoints not being written
: to the datafile headers. (O7 Concepts 24-4)
 

: I read once (but cant find the reference now) that one thing that is
: different during a Hot Backup is that the entries written to the on line
: log files are larger than usual. Basically instead of writing block offset
: references of changed data V7 must write the entire changed block.
 

: Can anyone confirm that this is a) still true for 7.1.3 and b) was ever
: true?
: --
: Regards,
 

: Lee E. Parsons
: Systems Oracle DBA lparsons_at_world.std.com

Yup, always was true and still is. Oracle writes the entire db block to disk during on line tablespace backups, basically because Oracle does not know how big your operating system i/o will be when you use your favourite backup command (dd,cpio,tar,dump etc) - if your system i/o size is less than the Oracle block size, curruption could occur during recovery if Oracle were to use the standard logical record redo structure.

For this reason, it it NOT a good idea to set ALL tablespaces in online backup mode at the same time - do them one at a time as they get backed up - unless your database is pretty quiet when this operation takes place.

Regards, Received on Mon Apr 24 1995 - 00:00:00 CEST