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38_ALUA_Configuration
Implicit ALUA comes into play in an HA ESOS setup, or when you have multiple paths (targets) and want to control, or at least advise/hint at the paths you'd like the initiators to use. Implicit ALUA differs from explicit ALUA in that the paths, via target groups, have their state advertised / sent to the initiators. Its up to the initiators to ultimately choose the correct path for I/O based on the information given from the ESOS SCST targets, and the target group policies are only set by the SCST on the ESOS (target side). In contrast, with explicit ALUA, the initiators can control the target group policy states by using a SCSI command (sent from the host/initiator).
In ESOS you'll configure ALUA by creating "device groups" and then under these device group(s), you'll have a set of target groups, states, and named targets. Typically the ALUA states are managed by the "SCST" cluster resource agent (part of ESOS). In the following sections, you'll see how to configure the device groups, add targets, and create target groups.
The ALUA target groups and states can be grouped by different sets of SCST devices, or they can be grouped all into one device group (typical). In the TUI, choose the add-device-group function (ALUA -> Add Device Group) to create a new ALUA device group. Next you'll need to add the SCST devices to the newly created device group: ALUA -> Add Device to Group
Repeat to add all of the SCST devices into your device group.
Next you'll need to create target groups on your ESOS host. In a typical environment, you might have a target group for all of your "local" targets, that is, targets directly on this ESOS host. The other target group would be for "remote" targets, those targets on another ESOS machine. Create these using the TUI (select the device group created above): ALUA -> Add Target Group
Choose a unique target ID for the "local" and "remote" target groups. When you create the ALUA configuration on the second ESOS host (assuming two in a cluster), reverse the local/remote target group IDs on that host.
Finally, you can add the SCST targets (ALUA -> Add Target to Group) to the newly created target groups. In the local target group, you'll add all of the SCST target names for target's that are on the ESOS machine. In the remote target group, add the target names of all the targets of the remote ESOS host. Set a unique relative target ID for each target you add (eg, 101, 102, etc.).
This completes the base ALUA configuration. The TUI does not provide functions for controlling the target group states directly, but this can be done on the command line in '/etc/scst.conf' file, or by using the scstadmin tool.
Again, the target group state are usually controlled by the SCST RA in ESOS. See the 42_Cluster_Configuration wiki page for information on configuring and using the SCST RA.