Planning an external storage system

To use Logical Units (LUs) of the external storage systems as Managed Disks (MDisks), several configuration requirements need to be followed to avoid performance and data integrity issues.

Use the following configuration guidelines that describe the system protocol for access to external storage systems LUs through multiple target ports.

External storage-system configuration-requirements

Follow the guidelines to configure the external storage systems arrays.
  1. Avoid splitting arrays into multiple logical disks at the external storage systems level. Where possible, create a single logical disk LU from the entire capacity of the external storage systems array.
  2. Depending on the redundancy requirements, create RAID-5 (RAID 5) arrays by using 5 - 8 data bits plus parity components that are, 5 + P, 6 + P, 7 + P, or 8 + P.
  3. Complete the appropriate calculations to ensure that your external storage systems are configured correctly.
  4. If any external storage system that is associated with an MDisk has the allowquorum parameter set to no, the chquorum command fails for that MDisk. Before you set the allowquorum parameter to yes on any external storage system, see IBM support website for external storage systems configuration-requirements.

Exporting external storage systems LUs to the system

Ensure that you are familiar and follow the following guidelines for exporting external storage systems LUs to the system.
  • When you define the system as a host object to the external storage systems, you must include all ports on all system nodes and system candidate nodes.
  • When you first create an external storage-system LU, you must wait until it is initialized before you export it to the system.
    • Do not present new LUs to the system until the array initialization format is complete. If you add a Logical Unit Number (LUN) to a storage pool before the array initialization format is complete, the storage pool goes offline. While the storage pool is offline, you cannot access the volumes that are in the storage pool.
      Attention: Failure to wait for the LUs to initialize can result in excessive system discovery time and an unstable view of the SAN.
  • When you export an external storage-system LU to the system, the LU must be accessible through all ports on the external storage system that are visible to the system.
  • The external storage-system LU must be mapped to all the system ports by using the same LUN for all external storage-system ports.
    Attention:

    The system uses the same LUN as the addressing method to identify an external storage-system LU through any access connection. Ensure that an external storage-system LU is mapped to all the system ports by using the same LUN for all external storage-system ports. Failure to do this mapping can cause loss of access to the associated MDisk.

After exporting the LU to the system, a system discovery might need to be requested to discover the new external storage-system MDisks and add them to the MDisks view.

External storage systems LU access through multiple ports

If an MDisk LU is accessible through multiple external storage-system ports, the system ensures that all nodes that access the LU coordinate their activity. The system also ensures that nodes access the LU through the same external storage-system port or group of ports depending on the specific access capabilities of the external storage system.

When the system can access an LU through multiple external storage-system ports, it uses the following criteria to determine the accessibility of these ports:
  • The system node is a member of a system.
  • The system node has Fibre Channel or iSCSI connections to the external storage-system port.
  • The system node successfully discovers the external storage-system LU.
  • Slandering does not cause exclusion of system node access to the MDisk through the external storage-system port.

An MDisk path is presented for all system nodes that meet these criteria.

MDisk port selection for external storage systems

When an MDisk is created, the system selects a single port or group of ports of the external storage-system ports to access the MDisk.

Table 1 describes the algorithm that the system uses to select the external storage-system port or group of ports.
Table 1. Storage-system port-selection algorithm
Criteria Description
Accessibility Creates an initial set of candidate storage-system ports that include the ports that are accessible by the highest number of nodes
Slandering Reduces the set of candidate storage-system ports to the ports that have lowest number of slanders
Preference Reduces the set of candidate storage-system ports to the preferred ports used by the external storage system
Load balance Selects the single port or group of ports from the set of candidate storage-system ports that has the lowest MDisk access count
After the initial external storage-system port-selection is made for an MDisk, the following events can cause the port selection algorithm to rerun:
  • A new node joins the system.
  • Requests for discovery of MDisks.
  • The external storage system changes its preferred port.
  • New external storage-system ports are discovered.
  • The currently selected external storage-system port becomes inaccessible.
  • Slandering causes exclusion of system node access to the MDisk through the external storage-system port.