07 and backups - do not trust backupexec

CLIFFORD ILKAY clifford_ilkay at dinamis.com
Sun Jan 17 14:20:39 EST 2010

On 01/17/2010 01:57 PM, David Swain wrote:
> Hi Doug,
> With MySQL it is basically the same. I agree with your basic premise,
> but would add a few more details:
> Even snapshots take a finite amount of time to perform (even if it's
> only seconds), during which time there could be some logical corruption
> creep in because of other operations being performed on the database.

This would be absolutely true for MyISAM tables. Is it the case for 
InnoDB tables too?

> The remedy for this is to lock all the tables to be backed up (in READ
> mode, if your system allows, which makes them still available for
> querying, just not for updating), make the snapshot and then unlock the
> tables to get them back into full service once the snapshot is complete.
> (Paranoia is an admirable quality in a database administrator... ;-) The
> disruption to service is minimal, but necessary because snapshots are
> still not instantaneous.

You don't need to lock tables in PostgreSQL to realize read-consistent 
backups due to the nature of how PG achieves read-consistency.

> The next step is to "realize" the snapshot and do a cold backup from
> there - and then test the backup to make sure it's viable. To me, a
> "backup" that has not been tested is only an *attempt* at a backup.
> Trust, but verify...

Good advice.

Clifford Ilkay
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Toronto, ON
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