On 03/07/2016 01:51 PM, Dan Ritter wrote:
On Mon, Mar 07, 2016 at 07:55:33PM +0100, Guus Snijders wrote:
Op 7 mrt. 2016 14:02 schreef "Jack Coats" <j...@coats.org>:
If that is the case, it should be great for 'i just erased my last weeks
work' problem, but disaster recovery would be a issue (or for any non-flat
file recovery, like databases that are backed up 'live' rather than exports
being backed up).
I guess I'm missing the point here, but in the case of the 'live'
databases; how is that different?
Isn't that restore case always the same? Or should I think more along the
lines of feeding the data back to the dbms and let the software itself care
about storing the data?
You have three options:
1. Stop the database, thus freezing transactions to the
filesystem. Make your backup. Restart the database.
Pros: consistent, works, as fast as any consistent backup can be.
Cons: stops your database.
2. Tell your database to dump a consistent copy of itself out to
a backup directory. Make the backup.
Pros: consistent, works, no database downtime.
Cons: on restore of a database, you MUST drop your existing
files and restore from the clean dump. Failure to do this will
cause your database to be inconsistent. Not all databases
support this feature. (But Postgres, MySQL/MariaDB, and Oracle
do.)
3. Don't stop the database, just make the backup.
Pros: fast, uncomplicated.
Cons: almost guarantees an inconsistent database and many
problems down the line. Don't do this.
Any backup system should be scriptable to do (1), by stopping
the database before invoking it and restarting afterwards, or
(2) by making the backup contingent on a database dump.
-dsr-
Some database, such as Oracle, have a mode where writes to the database
on disk can be paused and a backup can be made. In Oracle terms, this
is "Archive Log Mode"
-- Matt
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