Dell POWERVAULT MD3600I - Manual (Page 31)

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 basic assumption for the round-robin policy is that
the data paths are equal. With mixed host support, the data paths may have different bandwidths or different
data transfer speeds.
• Least queue depth with subset — The least queue depth with subset policy is also known as the least I/Os or
least requests policy. This policy routes the next I/O request to a data path that has the least outstanding I/O
requests queued. For this policy, an I/O request is simply a command in the queue. The type of command or the
number of blocks that are associated with the command are not considered. The least queue depth with subset
policy treats large block requests and small block requests equally. The data path selected is one of the paths in
the path group of the RAID controller module that owns the virtual disk.
• Least path weight with subset (Windows operating systems only) — The least queue depth with subset policy is
also known as the least I/Os or least requests policy. This policy routes the next I/O request to a data path that
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