Administrator Guide

Managing Front-End I/O Ports
Front-end ports connect an Storage Center directly to a server using SAS connections or to the Ethernet networks and Fibre Channel
(FC) fabrics that contain servers that use storage. iSCSI, FC, or SAS I/O ports can be designated for use as front-end ports.
NOTE: For Storage Manager clients connected to a Storage Center with a Data Collector: If aStorage Center is
connected to a server with a SAS front end, nothing related to that SAS connection will be visible in the servers view of
Storage Manager.
Front-End Connectivity Modes
Storage Center uses either legacy mode, virtual port mode, or ALUA port mode to transport data to servers that use SAN storage. In
legacy mode, front-end I/O ports are configured in pairs of primary and reserved ports. In virtual port mode, all ports are active, and if one
port fails the load is distributed between the remaining ports within the same fault domain. In ALUA port mode, volumes are mapped using
two paths, active and passive.
NOTE: In Legacy mode, reserve ports and primary ports reside on separate controllers, providing controller-level
failover only. Legacy mode does not provide port-level failover.
The front-end connectivity mode is configured independently for Fibre Channel and iSCSI. Both transport types can be configured to use
the same mode or different modes to meet the needs of the network infrastructure. For example, a Storage Center can be configured to
use virtual port mode for iSCSI and legacy mode for FC.
• The front-end connectivity mode for FC and iSCSI ports is initially selected during Storage Center deployment.
• After deployment, the front-end FC and iSCSI ports can be changed from legacy mode to virtual port mode.
• After FC and iSCSI ports are configured for virtual port mode, they cannot be changed back to legacy mode.
NOTE:
Use legacy port mode only if the network environment does not meet the requirements for virtual port
mode.
• The front-end connectivity mode for SAS front-end is always ALUA port mode and cannot be changed.
Virtual Port Mode
Virtual port mode provides port and controller redundancy by connecting multiple active ports to each Fibre Channel or Ethernet switch.
In virtual port mode, each physical port has a WWN (World Wide Name), and is also assigned an additional virtual WWN. Servers target
only the virtual WWNs. During normal conditions, all ports process I/O. In the event of a port or controller failure, a virtual WWN will move
to another physical WWN in the same fault domain. When the failure is resolved and ports are rebalanced, the virtual port returns to the
preferred physical port.
Virtual port mode provides the following advantages over legacy mode:
• Increased performance: Because all ports are active, additional front-end bandwidth is available without sacrificing redundancy.
• Improved redundancy: Ports can fail over individually instead of by controller.
• Simplified iSCSI configuration: Each fault domain has an iSCSI control port that coordinates discovery of the iSCSI ports in the
domain. When a server targets the iSCSI port IP address, it automatically discovers all ports in the fault domain.
ALUA Port Mode
Asymmetric Logical Unit Access (ALUA) provides port and controller redundancy for SAS front-end connections.
Volumes mapped to a server using SAS front-end also have port and controller redundancy. Volumes mapped over SAS are mapped to
both controllers. The volume mapping is Active/Optimized on one controller and Standby on the other controller. If the port or controller
fails on the active controller, the paths to the other controller become Active/Optimized. The mapping on the first controller switches to
Standby. When the port or controller recovers, the mapping to the first controller returns to Active/Optimized and the mapping to the
second controller returns to Standby status.
Legacy Mode
Legacy mode provides controller redundancy for a dual-controller Storage Center by connecting multiple primary and reserved ports to
each Fibre Channel or Ethernet switch.
NOTE: Legacy mode is not available on SCv2000 or SCv3000 series storage systems.
In Legacy mode, each primary port on a controller is paired with a corresponding reserved port on the other controller. During normal
conditions, the primary ports process I/O and the reserved ports are in standby mode. If a controller fails, the primary ports fail over to the
corresponding reserved ports on the other controller. This approach ensures that servers connected to the switch do not lose
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