HP-UX 11i v3 Native Multi-Pathing for Mass Storage (August 2012)

5
Native multi-pathing features
LUN WWID and DSF
The various lunpaths to LUN devices are correlated based on a world-wide unique identifier (WWID)
that is associated with each LUN device. Indeed, the host can determine that different lunpaths lead to
the same LUN only if a LUN can be uniquely identified by the operating system. The LUN WWID is
based on SPC-3 commands Inquiry and VPD Inquiry Page 83h data.
Based on this LUN WWID, the mass storage subsystem creates one persistent DSF for each LUN
irrespective of the number of lunpaths to the LUN device. The association between a LUN WWID and
a LUN DSF is persistently stored on the host.
Using LUN WWID and LUN DSFs has the following advantages:
• LUN addressing scalability — Because LUN DSF uses opaque minor numbers, the mass storage
susystem can theoretically address up to 2
24
(16 million) LUNs. The pre-11.31i v3 limitations of 32k
lunpaths only exists for the legacy naming model.
• LUN agile addressing — A LUN DSF remains unaffected by certain SAN topology changes, such
as the LUN device being moved from one target port to another target port or, in the case of a
Fibre Channel port, a target N-Port ID changing.
• Abstraction of lunpaths management — If additional lunpaths (either through the addition of a new
SCSI controller or new SCSI target ports) are offered to a given LUN, or if existing paths
components to the same LUN are removed, the LUN DSF remains unaffected.
• One DSF per LUN irrespective of the number of lunpaths — Compared to 11i v2, the number of
LUN DSFs is no longer proportional to the number of lunpaths. It reflects the number of SCSI devices
connected to the host.
Some old SCSI devices do not support VPD Inquiry Page 83h data. For such devices, the mass
storage subsystem creates one LUN DSF per lunpath to the LUN device.
For more information on persistent DSFs, see the
HP-UX 11i v3 Mass Storage Device Naming white
paper.
Dynamic I/O load distribution
The mass storage subsystem dynamically distributes the I/O load across the available lunpaths to a
LUN according to a settable policy. This distribution is transparent to the application generating the
I/O load.
Dynamic I/O load balancing leads to significant improvements in I/O throughput. For more
information, see the
HP-UX 11i v3 Mass Storage I/O Performance Improvements white paper.
To aid in optimal resource utilization, the SCSI stack supports several load balancing policies and
tunables settable with scsimgr. For instance, disk devices use the round-robin load balancing policy
as the default policy. However, administrators have the flexibility to change and tune the default load
balancing policy of LUNs using scsimgr.
The sar command has been enhanced to display details about the I/O performance on each lunpath.
The scsimgr command shows the I/O distribution based on both the number of I/O operations and
the amount of data transferred for each lunpath.
Policies supported for disk devices
Disk devices support the following I/O load balancing policies: