Barrel roll is not used on the disk based system so can be ignored in essentially all cases.
BARREL sets the mode of the barrel roll. The barrel roll is designed to protect data against bad recording tracks. Within each head group of 8 tracks, or in some modes, within a pair of head groups, the data for each ``track'' is actually recorded first on it's assigned track, then the next frame (20,000 data bits) is recorded on the next track, and the next frame on the next track and so forth. After 8 or 16 frames, depending on mode, the track is back on it's original assigned head. With this happening, if a recorder track is bad, some of all channels in the roll group are lost, but no channel is lost completely. Thus there is just a drop in sensitivity rather than a distortion of the information being measured. Very nearly all users should take the default of roll_auto. The options for BARREL are:
Barrel roll should be ok for PCFS systems (VEX files). In Dec 2000 the definition of barrel rolling has been updated to reflect the discovery that the current (VLBA) practice is time reversed with respect to the documentation.
Barrel roll is turned off for disk systems.