Important partitioning considerations with dual-slot and multi-adapter configurations

Logical partitions can own physical I/O resources. Physical I/O resources are assigned to logical partitions at the slot level. Assigning a slot to a logical partition enables the operating system that runs in the logical partition to control the functionality of the I/O resource and power for that slot. When the operating system powers a slot on or off, the physical I/O resource is powered on or off.

In some I/O configurations, the functionality of an adapter or I/O resource depends on two or more physical slots. For example, if you have a double-wide RAID adapter (FC 2053, 2054, or 2055) that take up two adjacent adapter slots or two separate RAID adapters paired together, both physical slots must be assigned to the same logical partition. For example, if you install the FC 2053, 2054, or 2055 adapter in slot 2, then the adjacent slot 3 cannot be used to install another adapter even though the slot 3 is reported as empty. It is important to understand the wanted configuration and function being provided before completing the logical partitioning and activation of the resources involved.

Multi-initiator and high availability

The terms multi-initiator and high availability (HA) refer to connecting multiple adapters (typically two adapters) to a common set of disk expansion drawers for increasing availability. This configuration is also referred to as Dual Storage IOA configuration. This type of connection is commonly done in either of the following configurations:

HA two-system configuration

An HA two-system configuration provides a high-availability environment for system storage by enabling two systems or partitions to have access to the same set of disks and disk arrays. This feature is typically used with the IBM®PowerHA® SystemMirror®. The IBMPowerHA SystemMirror software provides a commercial computing environment that ensures that mission-critical applications can recover quickly from hardware and software failures. The support for this configuration is operating system dependent.

HA single system configuration

An HA single system configuration provides for redundant adapters from a single system to the same set of disks and disk arrays. This feature is typically referred to as Multi-Path I/O (MPIO). MPIO support is part of the operating system support and can be used to provide a redundant IBM SAS RAID controller configuration with RAID protected disks.

Auxiliary write cache adapter

The auxiliary write cache (AWC) adapter provides a duplicate, nonvolatile copy of write cache data of the RAID controller to which it is connected.

Protection of data is enhanced by having two battery-backed (nonvolatile) copies of write cache, each stored on separate adapters. If a failure occurs to the write cache portion of the RAID controller, or the RAID controller itself fails in such a way that the write cache data is not recoverable, the AWC adapter provides a backup copy of the write cache data to prevent data loss during the recovery of the failed RAID controller. The cache data is recovered to the new replacement RAID controller and then written out to disk before resuming normal operations.

The AWC adapter is not a failover device that can keep the system operational by continuing disk operations when the attached RAID controller fails. The system cannot use the auxiliary copy of the cache for runtime operations even if only the cache on the RAID controller fails. The AWC adapter does not support any other device attachment and performs no other tasks than communicating with the attached RAID controller to receive backup write cache data. The purpose of the AWC adapter is to minimize the length of an unplanned outage, due to a failure of a RAID controller, by preventing loss of critical data that might have otherwise required a system reload.

It is important to understand the difference between multi-initiator connections and AWC connections. Connecting controllers in a multi-initiator environment refers to multiple RAID controllers connected to a common set of disk enclosures and disks. The AWC controller is not connected to the disks, and it does not perform device media accesses.

The RAID controller and the AWC adapter each require a PCI bus connection and are required to be in the same partition. The two adapters are connected by an internal connection. For the planar RAID enablement and planar auxiliary cache features, the dedicated connection is integrated into the system planar.