Administrator Guide

Virtual disk recovery

You can use the Edit host server-to-virtual disk mappings feature to recover data from the backup virtual disk. This functionality enables

you to unmap the original source virtual disk from its host server, then map the backup virtual disk to the same host server.

Ensure that you record the LUN used to provide access to the source virtual disk. You need this information when you define a host

server-to-virtual disk mapping for the target (backup) virtual disk. Also, be sure to stop all I/O activity to the source virtual disk before

beginning the virtual disk recovery procedure.

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 reroutes 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.

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.

You can choose one of these load balance policies to optimize I/O performance:

• Round-robin with subset — The round-robin with subset I/O load balance policy routes I/O requests, in rotation, to each available

data path to the RAID controller module that owns the virtual disks. This policy treats all paths to the RAID controller module that

owns the virtual disk equally for I/O activity. Paths to the secondary RAID controller module are ignored until ownership changes. The

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