The Effect of Device Replacement on Agile Device Files

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Abstract
The agile naming scheme for mass storage devices was introduced in HP-UX 11i v3, which addressed
the limitations of the device naming scheme used in previous releases of HP-UX 11i. This paper
explains how online device replacements effect agile device files. In addition, this paper is intended
for system administrators who have basic knowledge of mass storage device naming on HP-UX 11i
v3.
Understanding device discovery
A typical storage device has multiple storage ports (target ports) with one or more Logical Units (LUs)
behind these ports. LUs are identified using logical unit names (also referred to as the World Wide
Identifier) obtained from Device Identification VPD INQUIRY (page 0x83). An LU is presented by a
target port using a Logical Unit Number (LUN). A unique LUN is assigned to each LU on each target
port. An LU can be assigned a different LUN on each port when presenting the LU to the host. In
other words, the LUN is only unique within the scope of a target port. The LU names (WWID) are
unique and never change. HP-UX 11i v3 uses this name to uniquely identify an LU and paths to an LU
when it is presented from multiple target ports.
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In HP-UX, a lunpath comprises a controller path, target port, and the LUN. One or more lunpaths can
be associated with a logical unit, and the logical unit itself is associated with an agile device file. For
example:
0/1/0/0 Å controller path
0/1/0/0.0x0x500508b3009097f9 Å target path
0/1/0/0.0x0x500508b3009097f9.0x4001000000000000 Å lunpath
When an LU is presented to the host from multiple target ports, multiple lunpaths are associated with
the LU. The storage stack assigns a handle to each LU name (WWID) and uses this handle to
generate an agile device special file.
Figure 1 shows a single path to logical LUN 0x4001000000000000. The LUN is mapped to WWID
0x600508b3009097f0421745217a6c0057 and HP-UX represents it as disk1 in the agile name
space.