When a program requests a hard guarantee, it asserts that nothing, not even data integrity, should interfere with data transfer. A hard guarantee can be given only when
the SCSI controller or controllers that attach the real-time subvolume have only disks attached to them--no tapes or other nondisk devices
I/O to a non-disk device can delay disk data transfer.
sector remapping in the drive firmware, as well as any device driver retry and correction mechanisms, is disabled
Error retry can introduce unpredictable delays in data transfer.
When your program requests I/O under a hard guarantee, any device error is returned directly to the program. No effort is made to retry the failure. If the drive contains a bad sector, the bad sector is read and returned with no indication of error.