Re: Hot backups vs. Offline Backups

proberts_at_isc901.jsc.nasa.gov (Paul Roberts) wrote:

>HOWEVER, if you export the database, Oracle guarantees that you will
>have a copy of the database contents which is internally consistent.
>You can back this file up whenever you want.

I disagree that the database contents will be internally consistent. Export reads the tables one at a time so if a transaction updates 2 tables and commits, where 1 table has already been read by the export and the other has not, the final export file will only show the update against the second table.

Test this by interactively using export and asking for table 1 to be exported, wait till it has been read, in another session update table 2, in the first session ask for table 2 to be exported.

When you import you will get the updated rows but if the export was read consistent you would not. The read consistency across all tables is required for a reliable backup. Export is only read consistent for each table individually.

The only way this will work is if you prevent writes to the table during export which may not be acceptable.

--
All opinions expressed above are mine, and not my employer 
Russell McDonald  e-mail mcdonaldr_at_logica.com
Logica UK Limited  http://www.logica.com/