Switched logical unit

Switched logical units(LUNs) allows data that is stored in the independent disk pool from logical units that are created in an IBM® DS8000®, SAN Volume Controller, or Storwize® to be switched between systems providing high availability. Switchable LUNs are a set of disk units in an independent disk pool that is controlled by a device cluster resource group and can be switched between nodes within a cluster. When switched LUNs are combined with IBM i clusters technology, you can create a simple and cost effective high availability solution for planned and some unplanned outages.

A group of systems in a cluster can take advantage of the switchover capability to move access to the switched logical unit pool from system to system. A switchable logical unit must be in an IBM System Storage® connected through a storage area network. When the independent disk pool is switched, the logical units within the IBM storage are reassigned from one system to another.

The benefit of using switched LUNs for data resiliency lies in their operational simplicity. The single copy of data is always current, meaning there is no other copy with which to synchronize. No in-flight data, such as data that is transmitted asynchronously, can be lost, and there is minimal performance overhead. Role swapping or switching is relatively straight forward, although you might need to account for the time that is required to vary on the independent disk pool.

Another key benefit of using switched LUNs is zero-transmission latency, which can affect any replication-based technology. The major effort that is associated with this solution involves setting up the direct-access storage device(DASD) configuration, the data, and application structure.

Limitations are also associated with the switched LUN solution. First, there is only one logical copy of the data in the independent disk pool. This can be a single point of failure, although the data should be protected by using RAID 5, RAID 6, RAID 10, or mirroring. The data cannot be concurrently accessed from both hosts. Things such as read access or backup to tape operations cannot be done from the backup system. Certain object types, such as configuration objects, cannot be stored in an independent disk pool. You need another mechanism, such as periodic save and restore operations, cluster administrative domain or logical replication, to ensure that these objects are maintained.

Another limitation involves hardware associated restrictions. An example would be outages that are associated with certain hardware upgrades. The independent disk pool cannot be brought online to a system with a previous release. With this in mind, up-front system environment design and analysis are essential.

See PowerHA® supported storage servers for more information on the storage technologies that are provided by IBM.