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50 Using Your RAID Enclosure
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 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.
Ownership of a virtual disk is moved from the preferred controller to the
second controller (also called the alternate controller) when the preferred
controller is:
• Physically removed
• Updating firmware
• Involved in a fatal event that caused failover to the alternate controller
Paths used by the preferred controller 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.
When a RAID Controller Module Is Replaced or Removed
When one RAID controller module fails, or is removed from the RAID
enclosure, failover software on the host server directs the alternate RAID
controller module to assume all controller responsibilities for the storage
array. The virtual disks owned by the failed or removed RAID controller
module are failed over to the surviving controller and no data is lost.
NOTE: For Linux kernels, stop and then restart the SMagent after moving a
controller offline, replacing a controller, or configuring iSCSI connections.
Because host-based multi-path software is configured for automatic failback,
the data paths and virtual disks are restored to their original RAID controller
module after the RAID controller module is replaced.
Automatic failback occurs under the following conditions:
• Data is protected by dual redundant paths to the host
server
.
•The host
server
has multi-path failover software installed, and it is
configured for auto failback.