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

Media based errors

If the SCSI stack detects the LUN disk device could not commit a write from its cache to the media,

then it freezes the I/O traffic to the disk to prevent data corruption. When the error condition on the

LUN has been cleared, the system administrator must run

scsimgr enable –D lun_DSF to resume

I/O operations on the LUN.

Active-Passive device support

Active-passive arrays are supported if any of the following conditions are true:

• The vendor has developed and provided an Active/Passive Switch (APSW) plug-in module on HP-

UX 11i v3. For more information, see the

Writing Active white paper for

more information.

/Passive Switch Plug-Ins

• The disk array supports the T10 Asymmetric Logical Unit Access (ALUA) standard. For more

information, see SPC-3 Section 5.8.

• The disk array is now active-active symmetric and no loss of performance will occur on a host

performing multi-path load balancing simultaneously via different paths to the same LUN.

• The LUN or disk array is connected to a single host OS instance, the load_bal_policy attribute

is set to preferred_path or pref_tport, and the array is in auto-trespass mode.

SAN dynamic discovery and reconfiguration

SCSI devices dynamic discovery

At boot time, SCSI devices connected to the host are automatically discovered and corresponding

SCSI components are instantiated by the operating system. Once the system is up and running,

administrators can add new SCSI devices, disconnect existing SCSI devices, or replace path

components, such as a failing Fibre Channel HBA. Certain topology change events are automatically

detected by the mass storage subsystem while others require the system administrator’s intervention.

The mass storage subsystem asynchronously detects some topology changes in the SAN. For instance,

the addition of a Fibre Channel port to a Fibre Channel switch triggers a Fibre Channel Request

Status Change Notifications (RSCN) that is automatically detected by the SCSI stack. Upon receiving

the RSCN, the SCSI services automatically discover the LUNs connected to the newly added target

port.

However, some SAN topology changes are not automatically detected by the operating system. They

require the administrator to run ioscan. For instance, after adding Parallel SCSI LUNs, the

administrator must run ioscan to discover the newly added LUNs.

Both automatic SCSI device discovery and the ioscan command have the following results:

• The mass storage subsystem registers the corresponding paths components in both the legacy and

the agile I/O tree views.

• Legacy lunpaths DSF and LUN DSFs are automatically created to make the new devices readily

accessible to applications.

• The mass storage subsystem makes the new lunpaths available to I/O traffic with no interruption to

applications, and, in the case of automatic device discovery, without requiring special actions from

the system administrator.

The complexity of the SAN components under the LUN is hidden to the administrator:

• The dynamic discovery of components is transparent to the administrator. If needed, running

ioscan (without the –k option) automatically creates the new LUN DSFs.