Dell PowerVault MD3820f - Manual (Page 33)

Multi-Path Software
Multi-path software (also referred to as the failover driver) is the software resident on the host server that
provides management of the redundant data path between the host server and the storage array. For the
multi-path software to correctly manage a redundant path, the configuration must have redundant iSCSI
connections and cabling.
The multi-path software identifies the existence of multiple paths to a virtual disk and establishes a
preferred path to that disk. If any component in the preferred path fails, the multi-path software
automatically re-routes I/O requests to the alternate path so that the storage array continues to operate
without interruption.
NOTE: Multi-path software is available on the MD Series storage arrays resource DVD.
Preferred And Alternate Controllers And Paths
A preferred controller is a RAID controller module designated as the owner of a virtual disk or disk group.
The preferred controller is automatically selected by the MD Storage Manager when a virtual disk is
created. You can change the preferred RAID controller module owner of a virtual disk after it is created. If
a host is connected to only one RAID controller module, the preferred owner must manually be assigned
to the RAID controller module that the host can access.
Ownership of a virtual disk is moved from the preferred controller to the secondary controller (also called
the alternate controller) when the preferred controller is:
• Physically removed
• Updating firmware
• Involved in an event that caused failover to the alternate controller
Paths used by the preferred RAID controller module to access either the disks or the host server are
called the preferred paths; redundant paths are called the alternate paths. If a failure causes the preferred
path to become inaccessible, the storage array automatically uses the alternate path to access data, and
the enclosure status LED blinks amber.
Virtual Disk Ownership
The MD Storage Manager can be used to automatically build and view virtual disks. It uses optimal
settings to stripe the disk group. Virtual disks are assigned to alternating RAID controller modules when
they are created. This default assignation provides a simple means for load balancing the workload of the
RAID controller modules.
Ownership can later be modified to balance workload according to actual usage. If virtual disk ownership
is not manually balanced, it is possible for one controller to have the majority of the work, while the other
controller is idle. Limit the number of virtual disks in a disk group. If multiple virtual disks are in a disk
group, consider:
• The impact each virtual disk has on other virtual disks in the same disk group.
• The patterns of usage for each virtual disk.
• Different virtual disks have higher usage at different times of day.
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