Specifications

2–10 Controller and Host Concepts
During read-ahead caching, the controller anticipates subsequent read requests and
begins to prefetch the next blocks of data from the disks as it sends the requested read
data to the host. This is a parallel action. The controller notifies the host of the read
completion, and subsequent sequential read requests are satisfied from the cache
memory. By default, read-ahead caching is enabled for all disk units.
Write-Through Caching
When the controller receives a write request from the host, it places the data in its
cache module, writes the data to the disk drives, then notifies the host when the write
operation is complete. This process is called write-through caching because the data
actually passes through—and is stored in—the cache memory on its way to the disk
drives.
Write-Back Caching
Write-back caching improves the subsystem’s response time to write requests by
allowing the controller to declare the write operation “complete” as soon as the data
reaches its cache memory. The controller performs the slower operation of writing the
data to the disk drives at a later time.
By default, write-back caching is enabled for all units. In either case, the controller
will not provide write-back caching to a unit unless the cache memory is non-volatile,
as described in the next section.