IBM TotalStorage DS6000 Series - Manual (Page 367)

Appendix B. Using the DS6000 with iSeries 343
Prior to multipath being available, some customers used OS/400 mirroring to two sets of
disks, either in the same or different external disk subsystems. This provided implicit
dual-path as long as the mirrored copy was connected to a different IOP/IOA, BUS, or I/O
tower. However, this also required two copies of data. Since disk level protection is already
provided by RAID-5 or RAID-10 in the external disk subsystem, this was sometimes seen as
unnecessary.
With the combination of multipath and RAID-5 or RAID-10 protection in the DS6000, we can
provide full protection of the data paths and the data itself without the requirement for
additional disks.
Avoiding single points of failure
In Figure B-22, there are fifteen single points of failure, excluding the iSeries itself and the
DS6000 storage facility. Failure points 9-12 will not be present if you do not use an
Inter
Switch Link
(ISL) to extend your SAN. An outage to any one of these components (either
planned or unplanned) would cause the system to fail if IASPs are not used (or the
applications within an IASP if they are).
Figure B-22 Single points of failure
When implementing multipath, you should provide as much redundancy as possible. As a
minimum, multipath requires two IOAs connecting the same logical volumes. Ideally, these
should be on different buses and in different I/O racks in the iSeries. If a SAN is included,
separate switches should also be used for each path. You should also use Host Adapters in
different I/O drawer pairs in the DS6000. Figure B-23 on page 344 shows this.
6. Port
7. Switch
8. Port
1. IO Frame
2. BUS
3. IOP
4. IOA
5. Cable
9. ISL
13. Cable
10. Port
11. Switch
12. Port
14. Host Adapter