FC group

Each pool contains its own configuration for FC. It is assigned to a particular pool and can be used on any machine where a pool is imported. However, FC targets are local to a particular machine and each pool contains mapping of FC targets that has to be used on a particular machine. When the pool is imported on a machine where targets are not assigned to a pool configuration, it is possible to specify them after the pool import.

FC configuration consists of groups that define which volumes are available on given ports to configured initiators. Two type of groups are available: a public group and initiator groups. Public group allows any initiator connected to configured ports to access LUNs assigned to this group. Public group is present on a pool by default and cannot be removed or created. Initially, it didn’t have any volumes or ports assigned, so nothing is available until it is configured manually. Because this group allows to connect any initiator, it is not possible to assign initiators to this group. The second type of FC group is an initiator group. By using this group it is possible to define which initiators can connect to LUNs assigned to a group. Initiator that is not assigned to a FC group won’t be able to connect through ports to LUNs. It is possible to configure many initiator groups to have different access configurations to volumes through FC targets. It is possible to define alias for each initiator group that allows easier identification of the group purpose.

In general, groups gather set of: ports, volumes and initiators. LUNs added to a group define which volumes are available in this group. Ports assigned to a group define on which ports it is possible to connect to LUNs in a particular group. Finally, initiators (in case of an initiator group) define which initiators (ports of remote machines) will be able to connect to LUNs in the group using ports that are assigned to the group. For example, to allow initiators Ini0 and Ini1 access volume Vol-01 through ports P0 and P1 it is required to create an initiator group with assigned ports: P0 and P1, next add volume Vol-01 and initiators Ini0 and Ini1 to this group. It is possible to assign the same volume, initiator or port to more than one group. However, there are some limitations to the configuration that the system won’t allow to be applied:

1. The same target cannot be assigned to two groups that share a set of initiators.
2. Due to the rule above, the same initiator cannot be assigned to two groups that share a set of ports.
3. The target assigned in a public group cannot be used by an initiator group and the other way around, the target assigned to an initiator group cannot be used in public group.
4. The target can be assigned to only one pool - the same port cannot be used in two groups that belong to different pools. If the target is used in an active pool and another pool that also uses this target is imported, the group using a conflicting target will be deactivated upon import.

Moreover, a volume used by iSCSI cannot be assigned to any FC group and the other way around.

A created group can be modified at any point in time. It is possible to assign or remove initiators, ports or volumes. Volumes assigned to groups can be modified, however, it is advised to be careful because in some cases the connected initiator may lose access to the volume during this operation.

It is possible to deactivate any FC group. When a group is inactive, configuration that is represented by that group is not applied to ports thus LUNs are not available to initiators from a given group. A group can be deactivated either manually or by the system in case of configuration conflicts. Configuration conflicts are possible mainly during a foreign pool import. A group is deactivated on the imported pool in case of the following conflicts:

1. Target used by the pool is already used by other active pool.
2. One of the LUNs uses SCSI ID that is already used by FC or iSCSI LUN on other pool.

A group that was deactivated due to conflict can be activated manually after resolving that conflict by modifying the configuration.

A bit more explanation is required for the SCSI ID uniqueness. This LUN identifier consists of 16 characters, however two SCSI IDs that have the same first 8 characters are considered conflicting. It is due to the way some initiators read those identifiers. Some initiators honor only those first 8 characters of SCSI ID, which could lead to issues if two LUNs would have this part of SCSI ID the same. However, in most cases you don’t have to worry about this setting because the system assigns a unique SCSI ID to the volume based on it’s name and time stamp of creation. When a SCSI ID is not specified, the one assigned to volume is used for a LUN. It is recommended to use a default (generated by system) SCSI ID. To use the default value simply do not specify any SCSI ID during configuration of LUN.

FC targets assigned to groups require additional configuration to be used in a cluster environment. For detailed description of this configuration please refer to the Edit FC target properties section.