Image Backups

What's an image backup? What's the difference between an image backup and a standard file backup? Why would I use an image backup?

Image backups (aka sector backups) have been a fixture on mainframes for years. In general, an image backup focuses on sectors and is independent of the sectors' content. Therefore, an image backup contains information about partition tables, file tables (e.g., FAT, Master File Table—MFT), and the Master Boot Record (MBR). File backups are a more recent development. File backups contain information about files and file attributes. In a file backup, you can selectively restore individual files, whereas in an image backup, you attempt to restore an entire disk.

Both backup types offer distinct advantages. An image backup lets you boot to a set of 3.5" disks and restore the tape contents, thereby regenerating your hard disk. However, the size of the disk to which you restore must be at least equal to the size of the disk that you back up. (Best practice is to use disks of identical size.) In all cases, the disks should be low-level formatted to optimize the restore. You don't need to partition or format the recipient disk.

As you might expect, you can't perform an incremental image backup. Consequently, you should use image backups only for true disaster recovery and use file backups for individual file restoration. (Computer Associates' ARCserve was the first backup application to allow specific file restoration from an image backup, so exceptions exist to the rule about using image backup restorations.)