Accessing storage systems with multiple remote ports

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If a managed disk (MDisk) logical unit (LU) is accessible through multiple storage system ports, the system ensures that all nodes that access this LU coordinate their activity and access the LU through the same storage system port.

Monitoring LU access through multiple storage system ports

When the system can access an LU through multiple storage system ports, it uses the following criteria to determine the accessibility of these ports:
  • The system node is a member of a storage system.
  • The system node has Fibre Channel or iSCSI connections to the storage system port.
  • The system node has successfully discovered the LU.
  • Slandering has not caused the system node to exclude access to the MDisk through the storage system port.

An MDisk path is presented to the storage system for all system nodes that meet these criteria.

Storage-system port selection

When an MDisk is created, the system selects one of the storage system ports to access the MDisk.

Table 1 describes the algorithm that the system uses to select the storage system port.
Table 1. Storage system port selection algorithm
Criteria Description
Accessibility Creates an initial set of candidate storage-system ports. The set of candidate storage-system ports include the ports that are accessible by the highest number of nodes.
Slandering Reduces the set of candidate storage-system ports to those with the lowest number of nodes.
Preference Reduces the set of candidate storage-system ports to those that the storage system uses as preferred ports.
Load balance Selects the port from the set of candidate storage-system ports that has the lowest MDisk access count.
After the initial device port selection is made for an MDisk, the following events can cause the selection algorithm to rerun:
  • A new node joins the system and has a different view of the storage system than the other nodes in the system.
  • The detectmdisk command-line interface (CLI) command is run or the Discover MDisks management GUI function is used. The detectmdisk CLI command and the Discover MDisks function have the system rescan the Fibre Channel or iSCSI network. The rescan process discovers any new MDisks that might have been added to the system and rebalances MDisk access across the available storage system ports.
  • Error recovery procedures (ERPs) are started because a storage system has changed its preferred port.
  • New storage system ports are discovered for the storage system that is associated with the MDisk.
  • The storage system port that is currently selected becomes inaccessible.
  • Slandering has caused the system to exclude access to the MDisk through the storage system port.