How does an online backup work

[doom1701]

This has been one of those stupid questions keeping me awake at night. And while every DBMS that supports online backups probably works the same way, the smartest DBMS people I know are here.

Anyway, when I run an online backup of our ERP system database, what is really getting backed up? If I start my backup at 10pm, and the backup takes until 10:15pm, and the database is constantly being modified, is the resulting backup of the data from 10:15, 10pm, or somewhere in between?

 

[tamhas]

The backup will be as of 10pm. It takes a snapshot of the BI ... which you may perceive as a pause at the start of the session, and then as it goes along, before it lets any disk update modify the disk image from where you started, it takes a copy before the change so that is is all consistent as of the start.

 

[RealHeavyDude]

To add to it:

Generally the online is backing up the database blocks in sequential order. As soon as a database block is due for modification which has not been backed up the following happens:

• The online backup suspends the modification
• Jumps to that block and writes it into the backup
• The modification goes through and the online backup jumps back to where it was before

When it then reaches such a block the online backup knows that it has already been backed up and skips it.

Therefore you get a snapshot of the database with a time stamp of the beginning of the online backup.

HTH, RealHeavyDude.