Storage Multi-Pathing choices in HP-UX Serviceguard environments, August 2009

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availability to have more than one multi-pathing solution providing services for the same device or
LUN nor is it supported.
It might even be counterproductive having multiple multi-pathing solutions installed. DMP, SecurePath,
and PowerPath all try to provide multi-pathing to all devices they find during a discovery process,
therefore the administrator has to ensure that only one product is active for specific LUN or sometimes
a specific pair of controllers. Each of these solutions provides commands to reduce its scope on the
different levels of granularity (LUN, controller, or array). Please see the product specific
documentation referenced to in the “Multi-pathing product reference documentation” section above.
Multi-Pathing solutions on HP-UX 11i v3
The mass storage subsystem in HP-UX 11i v3 has been re-architected to increase configuration limits
and provide an enriched feature set. It is far beyond the scope of this white paper to discuss all the
features and benefits of it. Please see “Overview: Next Generation Mass Storage Stack” for an
introduction of the HP-UX 11i v3 mass storage subsystem and see the list at:
http://docs.hp.com/en/oshpux11iv3.html#Storage%20Area%20Management for additional information on
various areas of the HP-UX 11i v3 mass storage subsystem.
A new representation of mass storage devices called the “agile view” has been introduced along with
the new storage subsystem. The central idea of the agile view is that a DSF identifies a disk device or
LUN and not a specific path to a device as in HP-UX 11i v2. Paths to a device can change
dynamically, and multiple paths to a single device can be transparently treated as a single virtualized
path, with I/O distributed across those multiple paths.
These new DSFs which represent LUNs instead of LUN paths are also called “Persistent” DSFs in
contrast to the “Legacy” DSFs since they do not change even if the path to a LUN changes. The
Legacy DSFs, as known from HP-UX 11i v2, are still available on HP-UX 11i v3 for backward
compatibility. Users are encouraged to move to the Persistent DSFs wherever possible, because
Legacy DSFs are subject to HP-UX 11i v2 limits. The Persistent DSFs are stored in a different directory
structure:
 /dev/disk for block device files and
 /dev/rdisk for character device files.
Figure 5 shows how to map between Legacy and Persistent DSFs.
Figure 5: Map between persistent and legacy DSFs
# ioscan -m dsf /dev/dsk/c6t0d1
Persistent DSF Legacy DSF(s)
========================================
/dev/disk/disk87 /dev/disk/c6t0d1
# ioscan -m dsf /dev/disk/disk87
Persistent DSF Legacy DSF(s)
========================================
/dev/disk/disk87 /dev/dsk/c6t0d1
/dev/dsk/c8t0d1
Even though Legacy DSFs are subject to HP-UX 11i v2 limits (e. g. number of LUNs and LUN paths),
Native Multi-Pathing is available using the Legacy DSFs on HP-UX 11i v3 and is enabled per default.
The white paper: “HP-UX 11i v3 Native Multi-Pathing for Mass Storage” drills down on the multi-pathing
related features in the new mass storage subsystem.