HP-UX System Administrator's Guide: Overview HP-UX 11i v3 (B3921-90011, September 2010)

=====================================================
64000/0xfa00/0x2 disk HP 36.4GMAN3367MC
64000/0xfa00/0x3 disk HP 36.4GMAN3367MC
Legacy versus Agile Device Addressing
Beginning with HP-UX 11i version 3, mass storage devices are referenced by device instances
rather than by the hardware paths to the devices. This has many benefits over the previous
addressing scheme which associated a given device special file with the hardware path to a
device. Device hardware addressing for mass storage devices is now fluid, automatic, and
transparent. This has many benefits.
NOTE: For compatibility with previous HP-UX releases, the previous (legacy) device addressing
scheme for mass storage devices is still supported under HP-UX 11i version 3; therefore scripts,
configurations, and other uses of device special files you have previously created will continue
to work.
For transition purposes, you can use both legacy device addressing (using legacy device special
files) and agile device addressing (using persistent device special files) simultaneously, but to
take advantage of the many benefits of agile device addressing, and for future compatibility reasons,
you should transition to using persistent device special files going forward once all of the
underlying file systems and technologies that you use can support them.
Greater Configuration Stability
Agile device addressing allows for hardware paths to change between system boots (for example,
if a LUN is moved from one HBA to another while a server is shutdown) and for SAN
configurations to change without requiring changes to device special files (and therefore without
requiring changes to other configuration files). If you replace a disk associated with a persistent
device special file (the type of device special files that provide agile device addressing), you can
use the io_redirect_dsf command to update the persistent device special file to reference
the replacement disk. For details, see the manpage io_redirect_dsf(1M).
Scalability
Because of a limitation in device special file minor numbers, a server was previously limited to
256 bus instances. Using agile device addressing you can now address more than 256 bus instances.
Agile device addressing also allows you to address a greater number of LUNs. HP-UX 11i version
3 supports up to 16,384 LUNs.
Multi-pathing
Each LUN can have up to 32 physical I/O paths. HP-UX 11i version 3 automatically discovers
and configures new physical I/O paths to a LUN and balances data flowing through the various
paths to a given device using one of the following load-balancing policies:
closest_path
This load balancing policy selects a LUN based on its affinity with the
processor core issuing the I/O, minimizing latency.
cl_round_robin
This load balancing policy is applicable to HP cell-based platforms. The
LUN paths are selected in a round robin manner within the locality of
CPU on which the I/O was initiated, to ensure that memory access latencies
are optimized.
least_cmd_load
Directs I/O requests through the hardware path with the least outstanding
I/O requests.
round_robin
Cycles I/O requests through available hardware paths in round robin
fashion.
path_lockdown
Directs I/O requests through a single hardware path; specifically, the path
with the fewest outstanding I/O requests when the device is opened. This
54 Major Components of HP-UX