Note the model numbers of the drives in Figure 22. The models with a zero as the
last digit do not contain parity stripes.
Spreading parity across eight disk units can provide better performance in the
event of a disk unit failure, since the information required to dynamically rebuild
the data on the failed disk after replacement is accessed from an eighth of the
disk units, as opposed to one-fourth. The disk unit controller reads the parity and
data from the same data areas on 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 recreated, 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 subsystem 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.
Of course protection must be activated to allow concurrent maintenance to take
place. Use of concurrent maintenance is supported only to replace a failed drive
with the same size replacement drive.
Figure 23 on page 34 shows an example of a fully populated #5052 or #5058
when DPY is activated. Note that two sets are generated, each having all
members with parity stripes.
K1 K2 K3 K4 K5 K6 K7 K8
@0100 @0200 @0300 @0400 @2500 @2600 @2700 @2800
K9 K10 K11 K12 K13 K14 K15 K16
@1100 @1200 @1300 @1400 @1500 @1600 @1700 @1800
Mod
074
Mod
070 Set of
Parity
Mod
074
Mod
074
Mod
074
Mod
070
Mod
070
Parity
Stripes
34 AS/400 Disk Storage Topics and Tools
Figure 23. DPY: 16 drives (two DPY sets)
Notice that the last digit in the model number is a two. This is an indication that
the drive is a member of an 8 striped drive parity set. Note in Figure 22 that the
last digit on four of the DASD was a four. This is an indication that those DASD
are members of a parity set that has only four striped drives. The other drives
have full capacity available for customer data. In either case, no matter whether
there are four or eight drives that contain parity stripes, each DPY set must
provide the total capacity of one member of that set to store the parity information
on. For example, if the DPY set is made up of four, 8 GB DASD, parity stripes add
up to 8 GB of information for that set. If there were eight members (8 GB each) of
the DPY set, each containing parity stripes, the set still requires 8 GB of space to
store parity information for that set.

-----Original Message-----
From: MIDRANGE-L <midrange-l-bounces@xxxxxxxxxxxxxxxxxx> On Behalf Of Roberto José Etcheverry Romero
Sent: Wednesday, October 27, 2021 2:20 PM
To: Midrange Systems Technical Discussion <midrange-l@xxxxxxxxxxxxxxxxxx>
Subject: Re: Disk 4328-072 vs 4328-074

050 means that the disk has no RAID protection
070 072 074 etc mean that it is part of a parity set and the last digit references how much space is dedicated to the parity space (I cannot remember what %).


On Wed, 27 Oct 2021 at 14:31, Peter Dow <petercdow@xxxxxxxxx> wrote:

What are the differences between these two disk drives?

--
*Peter Dow* /
Dow Software Services, Inc.
909 793-9050
petercdow@xxxxxxxxx
pdow@xxxxxxxxxxxxxx /
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