What is a disk image?

A disk image is an exact representation of a hard disk or partitions of a hard disk. The image generally contains compressed data and only contains parts of the disk that are currently in use by the file system.

For a more detailed description, please read the following..

Sectors

Every computer hard disk is divided into addressable blocks called sectors.

Each sector is 512 bytes of data and each hard disk defines its partitions (drive letters in Windows) as a start sector, relative to the beginning of the hard disk, and length, the total number of sectors between the first and last sector in the partition.

File System

Each partition will usually be formatted into a file system; for Windows XP, this means either NTFS or FAT32.

Both of these file systems group together a fixed number of blocks to create a cluster, and a cluster is the smallest amount of disk space that Windows can read or write to.

All clusters that are currently being used by the file system are stored in a table, known as the file system bitmap, so by reading the bitmap we know which clusters are in use and which physical sectors of the hard disk stores the information.

Image

To create an image of the disk Reflect™ first reads the file system bitmap for each partition being imaged. it then reads the first used cluster from disk and compresses the data (if required) before writing to the image file. Each used cluster is read and written to the image file. After all the data is written an index is appended to the end of the file to reference the compressed data back to the original location on the disk.

To ensure that you can continue using windows whilst the image is being created, a special driver, 'pssnap.sys', creates a snapshot of the system at the beginning of the backup. Before Windows makes any changes to clusters that haven't yet been saved, 'pssnap.sys' copies the contents of the clusters and Reflect knows to use these copies for the image and not the modified disk contents. This enables the Image to contain only clusters at an exact point in time, thus if the image is restored the file system will be completely intact.