Multipath - IBM TotalStorage DS6000 Series Redbooks

Figure B-21 New logical volume shown on iSeries Navigator

Multipath

Multipath support was added for external disks in V5R3 of i5/OS (also known as OS/400
V5R3). Unlike other platforms which have a specific software component, such as
Device Driver
connections can be defined from multiple I/O adapters on an iSeries server to a single logical
volume in the DS6000. Each connection for a multipath disk unit functions independently.
Several connections provide availability by allowing disk storage to be utilized even if a single
path fails.
Multipath is important for iSeries because it provides greater resilience to SAN failures, which
can be critical to OS/400 due to the single level storage architecture. Multipath is not available
for iSeries internal disk units but the likelihood of path failure is much less with internal drives.
This is because there are fewer interference points where problems can occur, such as long
fiber cables and SAN switches, as well as the increased possibility of human error when
configuring switches and external storage, and the concurrent maintenance on the DS6000
which may make some paths temporarily unavailable.
Many iSeries customers still have their entire environment in the System ASP and loss of
access to any disk will cause the system to fail. Even with User ASPs, loss of a UASP disk
will eventually cause the system to stop. Independent ASPs provide isolation such that loss of
disks in the IASP will only affect users accessing that IASP while the rest of the system is
unaffected. However, with multipath, even loss of a path to disk in an IASP will not cause an
outage.
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DS6000 Series: Concepts and Architecture
(SDD), multipath is part of the base operating system. At V5R3, up to eight
Subsystem