Subject: Storage Network Management and Retrieval

Difficulty: Easy

Marks: 05M

Hot backup and cold backup are the two methods deployed for backup.

They are based on the state of the application when the backup is performed. In a hot backup, the application is up and running, with users accessing their data during the backup process. In a cold backup, the application is not active during the backup process.

The backup of online production data becomes more challenging because data is actively being used and changed. An open file is locked by the operating system and is not copied during the backup process until the user closes it. The backup application can backup open files by retrying the operation on files that were opened earlier in the backup process. During the backup process, it may be possible that files opened earlier will be closed and a retry will be successful. The maximum number of retries can be configured depending on the backup application. However, this method is not considered robust because in some environments certain files are always open.

In such situations, the backup application provides open file agents, these agents interact directly with the operating system and enable the creation of consistent copies of open files. In some environments, the use of open file agents is not enough. For example, a database is composed of many files varying sizes, occupying several file systems. To ensure a consistent database backup, all files need to be backed up in the same state. That does not necessarily mean that all files need to be backed up at the same time, but they all must be synchronized so that the data base can be restored with consistency.

Consistent backups of databases can also be done using a cold backup. This requires the database to remain inactive during the backup. Of course the disadvantage of a cold backup is that the database is inaccessible to user during the backup process. Hot backup is used in situations where it is not possible to shut down the database. This is facilitated by database backup agents that can perform a backup while the database is active. The disadvantage associated with a hot backup is that the agents usually affect overall application performance.