Maximum LUN Configuration and Considerations for HP-UX

Introduction
Over the years, there have been many advances in mass storage I/O interfaces and protocols, mass
storage devices, and server technologies:
• Link speeds many times faster (4GB Fibre Channel and 10 Gigabit Ether)
• Device data transfer speeds over ten times faster (Atrium 960 @ 160MB/s)
• LUN capacities greater than 2TB
• Disk storage requiring a merge of network and mass storage functionality
• Greater number of logical units and storage capacity required per server
• Higher scalability and application performance requirements
These advancements have put greater demand on the server I/O subsystem than ever before. HP-UX
mass storage scalability has been enhanced throughout different releases to meet these increasing
requirements.
This paper highlights the increases in the maximum number of LUN devices and LUN paths supported
with different releases of HP-UX as well as specific factors to consider for each release.
On releases prior to HP-UX 11i v3, a LUN path can be opened, accessed and closed via I/O or
control operations. For user access on pre-HP-UX 11i v3 a LUN path typically has at least one device
special file associated with it. HP-UX 11i v3 abstracts which path is being accessed for a given LUN
device, providing device special files which give access to the LUN device independently of any
specific path. See the “
HP-UX 11i v3 Mass Storage Device Naming” white paper for details
regarding the new HP-UX 11i v3 device special files.
Two cases of abstracted LUN paths used on releases prior to HP-UX 11i v3 occur in alternate path
failover functionality such as LVM’s PVLinks and in dynamic multi-pathing functionality such as
SecurePath. Dynamic multi-pathing refers to the capability to determine the various paths to a given
LUN device at runtime and to dynamically load-balance I/O operations across those paths. As a
result, all LUN paths that are accessed via a dynamic multi-pathing module are considered load-
bearing and hence “active” in the definitions below. Dynamic multi-pathing modules tend to include
alternate path failover functionality as well, but the key characteristic of such modules for this paper is
that they increase the number of “active” paths that exist on the system. HP-UX 11i v3 has dynamic
multi-pathing built-in to the operating system.
The following terms are used to describe LUN count maximums:
• Active LUN device: A LUN device with at least one Active LUN
• Active LUN path: A load-bearing path to a LUN device
• Open LUN path: An "Active LUN path" or a path that is open with only occasional I/O to
the LUN
• Visible LUN path: An "Open LUN path" or a path that is typically not open but is visible to
ioscan
Note that on releases prior to HP-UX 11i v3, all PVLinks paths to a given LUN device are considered
“Open” but only one of the paths is active at any point in time. Hence if a pre-HP-UX 11i v3 system
has 2000 LUN devices with 2 paths to each device for a total of 4000 LUN paths, all of which are
PVLink pairs, then the number of Active LUN paths would be 2000 and the number of Open and
Visible LUN paths would be 4000.
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