Abstract

A computer system includes a host computer, a mass storage subsystem and a backup subsystem for backing up information stored on the mass storage subsystem. The mass storage subsystem stores information on a series of tracks. A backup bit map includes a plurality of bits each associated with a respective one of the tracks and indicates the backup status of the track during a backup operation. Initially, during a backup operation, the bits associated with the tracks to be backed up will be set. Generally, the mass storage subsystem transfers information from the track to be backed up in order of the bits in the bit map, and after each track is backed up, it will clear the track's bit. However, when the host is to store information in the mass storage subsystem, it will determine whether the bit associated with the track in which the information is to be stored is set and, if so, enable the mass storage subsystem to back up the track out of turn, and to re-set the track's bit. After the information has been retrieved from the track, the bit associated therewith will be reset, and the host will store the information in the track. This allows the mass storage subsystem to ensure that the backed up information will correspond to the information as of the beginning of the backup operation, and in addition will allow the host to continue operations during the backup operation.