Discovering LUNs over the FCP protocol is possible for NPIV enabled devices and in DPM, all the FCP Adapters are NPIV enabled. The ability to discover LUNs over the Storage Area Network has many benefits. Programs can query the SAN about configuration in various elements of the SAN. DPM takes advantage of the following commands supported by the FCP protocol
- Read zone members
- List LUNs
- Read Capacity & Device Identification
The actual LUN discovery happens with a World Wide Port Number (WWPN) used on an NPIV port (FCP Adapter). The port then sends out requests standardized in the FCP specifications to to the switch. There are sequence of port logins, process logins involved in the process of sensing the LUNs configured for a WWPN when used on the specific port. For the sake of conciseness we will avoid going into the port logins and process logins but only discuss the 3 commands listed above. Please be advised that the names of the commands are different from what it is defined as, in the FCP specification documents. The intention is to explain what each command does, rather than being a reference to the specifications.
SAN Configuration
A system(e.g., IBM z16) is connected to a storage system(e.g., DS8000, V7000) using one or more storage switches. The switches have ports that have fibre optic cables connecting to the system and to the storage system. The storage system also has ports where the fibre optic cables plug into. The port on the system is called the initiator port and the port on the storage system is called the target port. Both the initiator and target ports have a WWPN associated with it and is globally unique. The switch and the storage system have software configuration that defines the rules on who connects to what. The switches have zones that group a set of initiator and target ports. The members of the zone are only allowed to communicate to each other and not to any members outside of the zone. A port can be part of the one or more zones. Multiple zones make up a zone set and a switch can have more than one zone sets. A zone set is designated as the active zone set and that controls which ports can communicate to each other.
On the storage system side, usually host groups is created with a group of initiator ports and the host group is associated with one or more LUNs created on the storage system. All these configuration make up the SAN and these enable the storage device attached to the operating system to use the WWPN as the identifier and access the required storage devices.
DPM Storage Management
DPM allows the administrators to create a storage group, which is ideally a group of volumes with additional attributes that define how many paths each partition needs to connect to these volumes. Once created, DPM takes care of communicating the requirements to the storage administrator (SMTP configuration required) and then validating that all SAN configuration is done to satisfy the request. The validation helps the administrators know that the access to storage resources is verified. DPM does periodic validation to check if the resources initially requested are still configured correctly. The results of the validation are indicated in the state of the storage group.
The following are the various states, a storage group can be in:
Pending – The storage group configuration is yet to be validated after the initial creation or after a modification. In this state, the storage group is sensed every 10 minutes.
Pending with mismatches – The storage resources configured do not exactly match the requested configuration. There may be some additional volumes configured, or the volumes are not uniformly configured across all the adapters. In this state, the storage group is sensed every 10 minutes.
Complete – The storage group configuration is validated successfully against the request. At this state, when the storage group is attached to a partition, it is guaranteed to have access to all the storage resources over the required paths. In this state, the storage group is sensed every 24 hours.
Incomplete – The storage group configuration that was previously validated has changed in the SAN. Some of the volumes may not be available anymore or the previously validated adapters are not configured to access the volumes anymore. Apart from this, the state of the adapter also influences the Incomplete state. If an adapter used by a partition to access resources of this storage group, has any exception, for e.g., error counters exceeded, then it is indicated to the user by turning the storage group to an Incomplete state. If a storage group is attached to an active or reserved partition when it is turning Incomplete, chances are the storage resources are not accessible over the required paths and this will be indicated to the user by turning the partitions to a degraded state. The Partition details panel, will have additional information about the reason for degrade. In this state, the storage group is sensed every 10 minutes.
It is also important to note that DPM will use the WWPNs of a storage group on all available FCP adapters to query the configuration. This is important to identify the list of candidate adapters that can be used to create vHBAs when the storage group is attached to partitions. When a WWPN of a storage group is attached to an active partition, it cannot be used to sense on other adapters. Thus, you might notice that a storage group that is attached to active partitions finishes sensing quickly than a storage group that is not attached or in pending state.
Connection Report
The connection report option in the Storage Group Details view will have the result of the latest sensing attempt and the timestamp when the last sensing happened. It provides a view of how the SAN configuration is done for the WWPNs of a storage group. It does not provide a view of how an active partition is accessing the storage resources. The intention of the connection report is to highlight where the configuration errors are, if any. The connection report will list all the adapters and the switches they connect to. It will also try to distribute the WWPNs over multiple switches for redundancy and give a view on how the WWPNs are configured over these switches.
Conclusion
LUN sensing is an important aspect in FCP Storage Groups, that provides information of how SAN configuration is done outside the system. The information exposed by DPM via the fulfilment state and the connection report should be used by the administrators to get a view on configuration and necessary actions taken. The number of WWPNs in a storage group and the number of FCP adapters in a system defines the amount of sensing requests DPM will drive into the SAN. It is recommended to complete storage configuration quickly, so that DPM queries the configuration less frequently.