Newsletter

July Definition of the Month

Backup.

Backups are defined as the transactions made at a given interval that creates a point-in-time image or copy of data (and in many cases the OS and applications suites) and holds it in the infrastructure, either on disk or tape drives.

There are three basic backup types, full, incremental and differential. These determine how much data is backed up for a given operation. During a full backup, the entire data set is backed up, whereas an incremental backup stores only data that has changed since the last backup. A differential backup includes all the changes since the last full backup, including any changes that might have been incorporated into recent incremental backups.

Full backups involve more time and media than incremental backups. However, restores will be faster with more frequent full backups. Backup policies, driven by recovery and availability requirements, help to set the schedules and the types of backups to be performed. Beyond the availability requirements, some backup and recovery policies may be determined based on legal, audit or regulatory requirements.

It is not uncommon to find all three strategies used in the same environment. A full backup might be made on, say, Sunday, with incrementals made Monday and Tuesday. A differential backup then is made Wednesday, with incrementals on Thursday, Friday, and Saturday. It is important to remember that when restoring a backup all related full, incremental, and differential backups are required.

In the traditional file-based backup method, data residing on an application host is backed up through that host. The host reads data from the attached storage into its memory. It then writes the data out to a locally attached storage device (disk or tape) or to another backup host over a network, which in turn, writes it to an attached storage device. In either case, the application host has to perform the reads and writes required for backup. This, in many cases, takes all the host's cycles, creating the need for a "backup window" during which the backup transactions take place.

However, using imaging technology, it is possible to create a point-in-time snapshot of every sector on a disk, and then write those sectors out to an image while the host continues to operate uninterrupted. In this way, the so-called backup window disappears, as all disk activity is done in the background and the users are able to access the system being backed up without interruption.