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.

Important: If a failure of either the RAID controller or the Auxiliary Cache occurs, the Maintenance Analysis Procedures (MAPs) for the service request numbers (SRNs) in the AIX® error log must be followed precisely.

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 SAS connection. For the Planar RAID Enablement and Planar Auxiliary Cache features, the dedicated SAS connection is integrated into the system planar.

If the AWC adapter itself fails or the SAS link between the two adapters fails, the RAID controller will stop caching operations, destage existing write cache data to disk, and run in a performance-degraded mode. After the AWC adapter is replaced or the link is reestablished, the RAID controller automatically recognizes the AWC, synchronizes the cache area, resumes normal caching function, and resumes writing the duplicate cache data to the AWC.

The AWC adapter is typically used in conjunction with RAID protection. RAID functions are not affected by the attachment of an AWC. Because the AWC does not control other devices over the bus and communicates directly with its attached RAID controller over a dedicated SAS bus, it has little, if any, performance impact on the system.

Figure 1. Example RAID and AWC controller configuration Example of RAID and AWC controller configuration