HP-UX System Administrator's Guide: Overview
(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 is the default load balancing algorithm
for serial devices (for example tape drives).
preferred_path
The I/O path set in the preferred_path attribute is preferably used
for I/O transfer. If this I/O path is not available or if the
preferred_path attribute was not set, any other path is selected
for I/O transfer. This policy is useful for certain disk arrays,
which may exhibit some performance degradation if I/Os are
transferred via several I/O paths to a LUN simultaneously.
pref_tport
This load balancing policy selects LUN paths from a preferred
list unless none are available (or defined), in which case other
LUN paths are selected.
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