If a managed disk (MDisk) logical unit (LU) is accessible
through
multiple controller device ports, the SAN Volume Controller ensures
that all nodes that access this LU coordinate
their activity and access the LU through the same controller device
port.
Monitoring LU access through multiple controller device
ports
When
the
SAN Volume Controller can
access an LU through multiple controller device ports, the
SAN Volume Controller uses
the following criteria to determine the accessibility
of these controller device ports:
- The SAN Volume Controller node
is a member of a cluster.
- The SAN Volume Controller node
has fibre-channel connections to the controller device port.
- The SAN Volume Controller node
has successfully discovered the LU.
- Slandering has not caused
the SAN Volume Controller node
to exclude access to the MDisk through the
controller device port.
An MDisk path is presented to the
cluster for all
SAN Volume Controller nodes
that meet these criteria.
Controller
device port selection
When an MDisk is
created, the SAN Volume Controller selects
one of the controller device ports to access
the MDisk.
Table 1 describes
the algorithm that the
SAN Volume Controller uses
to select the controller device port.
Table 1. Controller device port selection
algorithm| Criteria |
Description |
|---|
| Accessibility |
Creates
an initial set of candidate controller device
ports. The set of candidate controller device ports include the ports
that
are accessible by the highest number of nodes. |
| Slandering |
Reduces the
set of candidate controller device ports
to those with the lowest number of nodes. |
| Preference |
Reduces the
set of candidate controller device ports
to those that the controller device uses as preferred ports. |
| Load balance |
Selects the port from the set of candidate controller
device 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 cluster and has a different view of the controller
device than the other nodes in the cluster.
- The svctask
detectmdisk command-line interface (CLI) command is
run or the Discover MDisks SAN Volume Controller Console function
is used. The svctask detectmdisk CLI
command and the Discover MDisks function have
the cluster
rescan the fibre-channel network. The rescan discovers any new MDisks
that
might have been added to the cluster and rebalances MDisk access across
the
available controller device ports.
- Error recovery procedures
(ERPs) are started because a controller device
has changed its preferred port.
- New controller device ports
are discovered for the controller device that
is associated with the MDisk.
- The controller device port that
is currently selected becomes inaccessible.
- Slandering has
caused the SAN Volume Controller to
exclude access to the MDisk through the controller
device port.