Considerations for multipath disk units

 

There are many considerations when creating multipath disk units.

Multiple connections to the logical unit number (LUN) from the IOAs were implemented in V5R3. Up to eight connections can be defined from multiple IOAs on a system to a single LUN in the Enterprise Disk Storage. The Enterprise Disk Storage concurrently supports diverse host systems over diverse attachment protocols. Data storage is allocated among the attached host systems with the Enterprise Disk Storage Specialist, a Web-based interface. Each connection for a multipath disk unit functions independently. Several connections provide availability by allowing disk storage to be utilized even if a single path fails.

When you use multipath disk units, consider the implications of moving IOAs and multipath connections between nodes. You must not split multipath connections between nodes, either by moving IOAs between logical partitions or by switching expansion units between systems. If two different nodes both have connections to the same LUN in the Enterprise Disk Storage, both nodes might potentially overwrite data from the other node.

The following figure shows a logical partition configuration with multiple paths configured correctly. Partition 0 (P0) has multiple paths defined from two different IOAs to one LUN in the Enterprise Disk Storage. Partition 1 (P1) also has multiple paths defined from two different IOAs to a different LUN in the Enterprise Disk Storage. The configuration would be incorrect if each logical partition had defined connections to a single LUN.

The system enforces the following rules when you use multipath disk units in a multiple-system environment:

  • If you move an IOA with a multipath connection to a different logical partition, also move all other IOAs with connections to the same disk unit to the same logical partition.

  • When you make an expansion unit switchable, make sure that all multipath connections to a disk unit will switch with the expansion unit.

  • When you configure a switchable independent disk pool, make sure that all of the required IOAs for multipath disk units will switch with the independent disk pool.

If a multipath configuration rule is violated, the system issues warnings or errors to alert you of the condition. It is important to pay attention when disk unit connections are reported missing. You want to prevent a situation where a node might overwrite data on a LUN that belongs to another node.

Disk unit connections might be missing for a variety of reasons, but especially if one of the preceding rules has been violated. If a connection for a multipath disk unit in a system or basic disk pool is found to be missing during an IPL, a message is sent to the QSYSOPR message queue.

If a connection is missing, and you confirm that the connection has been removed, you can update Hardware Service Manager (HSM) to remove that resource. Hardware service manager is a tool for displaying and working with system hardware from both a logical and a packaging viewpoint, an aid for debugging IOAs, IOPs, devices, and for fixing failing and missing hardware. You can access Hardware Service Manager in System Service Tools (SST) and Dedicated Service Tools (DST) by selecting the option to start a service tool.

Multiple connections are not supported for a load source LUN in an Enterprise Disk Storage from an eServer™ i5 model.

 

Resetting multipath

Use the Start DASD Management Operation (QYASSDMO) API to remove the missing multipath disk units, resulting in only showing the active paths.

The message Event EV0D0401, Event Code 1E, Message ID CPI096E will be in the QSYSOPR message queue when there is a missing connection to a disk unit.

 

Parent topic:

Multipath disk units
Related information
Start DASD Management Operation (QYASSDMO) API