Internal Magnetic Media 343
Internal Magnetic Media
Redundant Array of Independent Disks (RAID) specifications that were published by the
University of California in 1987. The high-availability models with Device Parity Protection use
a technique similar to RAID-5 data-redundancy technology to protect data. Throughout this
documentation, RAID and RAID-5 are often referenced, and are for the most part,
synonymous with Device Parity Protection.
RAID-5
See “Device Parity Protection” on page 342 for a discussion on RAID-5 and Device Parity
Protection. RAID-5 protection is supported for all internal disks provided that it is supported
by the disk controller.
A minimum of four disk units of the same capacity are required for a valid RAID-5
configuration. Parity information can be spread across four or eight of the disk units in an
array and is automatically maintained as part of the RAID-5 protection feature. Internal disk
units of different technology (that is, different feature numbers), but of the same capacity, can
be either mirrored or RAID-5 protected.
Having parity spread across eight disk units gives better performance in the event of a disk
unit failure since the data required to dynamically rebuild the data on the failed disk is
accessed from an eighth of the disk units as opposed to a quarter. If one disk unit fails, it
cannot be used to read or write data. The disk unit controller then reads the parity and data
from the same data areas as the other disk units to dynamically rebuild the original data from
the failed disk unit to satisfy ongoing read requests. When data needs to be written, the
controller generates the parity information for the failed disk unit as if it were still operating.
As far as the AS/400 system is concerned, the disk units continues to respond to I/O even
though a single disk unit has failed.
A RAID controller is necessary when concurrent maintenance support is required.
Mirroring
Mirrored protection is a function that increases the availability of the AS/400 system in the
event of a failure of a disk-related hardware component. It can be used on all models of the
AS/400 system. Software support is a part of the Licensed Internal Code. Different levels of
mirrored protection are possible, depending on the hardware that is duplicated. Mirroring
involves duplicating disk-related hardware, such as a disk unit, disk controller, disk
input/output processor (IOP), or a bus. When a disk-related mirrored component fails, the
system remains available.
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