Re: Backup strategy

In article <3r4ici$guc_at_mother.usf.edu>, rife_at_aarlo.moffitt.usf.edu (Mike Rife) says:
>
>In article <3r47u6$6k8_at_homer.alpha.net>, sds_at_dataplusnet.com (Strategic Data Systems) says:
>>
>>I have followup question on the use of 'hot backups'.
>>
>>I am running Oracle 7.0.x on an IBM RS/6000 under AIX.
>>If the put my tablespace in backup mode, and then proceed to backup the
>>datafiles using my favorite UNIX backup utilitiy, do I get a 'good' backup
>>of the datafile since there still is activity on the data file(s)?
>>
>>It would seem I would not, since I am backing up an ever changing file.
>>
>>==================================================================
>>Scott Overby sds_at_dataplusnet.com
>>SDS
>>Sheyboygan, WI 53082
>>==================================================================
>
>This is the only way to backup your tablespace files without shutting
>down your database. You do get a good copy of you files. Oracle handles
>the changing files.
>
>Have you tested your recovery procedure lately? Better now, then at
> 04:00AM with the production DB down!
Actually let me clarify if I might.

You do NOT get a consistent "good copy" of the file. How could you. A copy utility like TAR starts at block 0 of a file and copies blocks until the end. With the DB up and running it is highly probable that the copy misses blocks that are changing as a result of user transactions. This is OK.

Oracle "knows" the System Change Number ( SCN ) when the backup starts because of the "alter tablespace xxx begin backup". This info is recorded in either the redo log, the controlfile or both ... I can't remember and it really doesn't matter.

Oracle "knows" SCN when the backup finishes because of the "alter tablespace  xxx end backup".

Oracle therefor knows all changes that occurred to a tablespace while it was being backup up. On recovery when you lay down your inconsistent "bad backup copy" of the tablespace datafile(s) Oracle checks from the begining of the backup to ensure that all blocks are completely rebuilt....

works great !

Regards
GAG

Gerry A. Gilchrist ( GAG )			ObjectWorks Inc. 
						800, 100-4th Avenue S.W.  
						Calgary, Alberta 
						Canada T2P 3N2 
Tel: (403)237-7333
E-Mail:	gag_at_objectworks.com	WWW: http://www.objectworks.com 
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