Switchable and stand-alone independent disk pools

There are two basic environments in which you can take advantage of independent disk pools, a multisystem environment managed by a cluster and a single-system environment with a single system.

Independent disk pools in a multisystem clustered environment

A group of systems in a cluster can take advantage of the switchover capability to move access to the independent disk pool from system to system. In this environment, an independent disk pool can be switchable when it resides on a switchable device. A switchable device can be an external expansion unit (tower), an IOP on the bus shared by logical partitions, or an IOP or without an IOP hardware that is assigned to an I/O pool.

Hardware that does not have a physical IOP has a virtual logical representation of the IOP.

A switchable device that contains an independent disk pool can be switched automatically in the case of an unplanned outage, or it can be switched manually by administering a switchover.

Another option that can be leveraged in a multisystem environment is geographic mirroring. Geographic mirroring allows you to maintain two identical copies of an independent disk pool at two sites that are geographically separated. The independent disk pools at the separate sites can be switchable or dedicated.

Dedicated independent disk pools in a single-system environment

An independent disk pool in a single-system environment, with no clustering and no switchable devices, is said to be a dedicated, private, or stand-alone independent disk pool. While you cannot switch the access to the independent disk pool amongst systems in this environment, you can still isolate data in an independent disk pool, keeping it separate from the rest of the disk storage on the system. The independent disk pool can then be made available (brought online) and made unavailable (taken offline) as needed. This might be done, for example, to isolate data associated with a specific application program or to isolate low-use data that is only needed periodically. Dedicated independent disk pools might also be used to consolidate data from several small systems at branch offices to one or more larger systems at a central location, while still keeping the data separate for each branch.

Independent disk pools allow you to isolate certain maintenance functions. Then, when you need to perform disk management functions that normally require the entire system to be at DST, you can perform them by merely varying off the affected independent disk pool.

The following table compares dedicated independent disk pools and independent disk pools in a multisystem environment.

Consideration	Dedicated	Multisystem environment
Consideration	Single system	Multisystem cluster	Logical partitions in a cluster
i5/OS^® cluster required	No	Yes	Yes
Connectivity between systems	Not applicable	HSL loop	Virtual OptiConnect
Location of disk units	Any supported internal or external disk units	External expansion unit (tower)	IOP on shared bus
Switchability	No	Yes, between systems	Yes, between partitions
Switchable entity	None	Expansion unit	IOP

In a hardware switching environment, one node in the device domain owns it, and all the other nodes in the device domain show that the independent disk pool exists. In a geographic mirroring environment, one node at each site owns a copy of the independent disk pool. When an independent disk pool is created or deleted, the node creating or deleting the independent disk pool informs all the other nodes in the device domain of the change. If clustering is not active between the nodes, or a node is in the midst of a long running disk pool configuration change, that node will not update and will be inconsistent with the node rest of the nodes. Nodes must be consistent before a failover or switchover. Ending clustering and starting clustering will ensure that the configuration is consistent.

For more on switchable and dedicated independent disk pools, including example configurations for each of these environments, see Examples: Independent disk pool.

Parent topic:

Planning for independent disk pools