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.
Storage system port selection algorithm
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 MDisksmanagement 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.