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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 dual storage IOA connections and AWC connections.
Connecting controllers in a dual storage IOA 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 Isolation and
Recovery procedures for the System Reference Codes (SRCs) in the Service Action Log (SAL) or Product
Activity Log (PAL) 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, write out 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.
14
SAS RAID controllers for IBM i
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