Multi-Path Software; Preferred And Alternate Controllers And Paths; Virtual Disk Ownership; Load Balancing - Dell PowerVault MD3260i Administrator's Manual

You cannot use a snapshot repository virtual disk as a source virtual disk or as a target virtual disk in a virtual disk copy.
If you select the source virtual disk as the target virtual disk of a virtual disk copy, you must disable all snapshot virtual
disks associated with the source virtual disk.

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 Dense 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.

Load Balancing

A load balance policy is used to determine which path is used to process I/O. Multiple options for setting the load
balance policies let you optimize I/O performance when mixed host interfaces are configured.
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