Hard guarantees are possible only when the disks that are used for the real-time subvolume meet the requirements listed in the section "Hardware Configuration Requirements for GRIO" in this chapter.
Because of the disk configuration requirements for hard guarantees (see the section "Hardware Configuration Requirements for GRIO" in this chapter), incorrect data may be returned to the application without an error notification, but the I/O requests return within the guaranteed time. If an application requests a hard guarantee and some part of the system configuration makes the granting of a hard guarantee impossible, the reservation is rejected. The application can then issue a reservation request for a soft guarantee.
A soft guarantee means that the system tries to achieve the desired rate, but there may be circumstances beyond its control that prevent the I/O from taking place in a timely manner. For example, if a non-real-time disk is on the same SCSI controller as real-time disks and there is a disk data error on the non-real-time disk, the driver retries the request to recover the data. This could cause the rate guarantee on the real-time disks to be missed due to SCSI bus contention.