Specifications

4–12 Compaq StorageWorks HSG80 Array Controller ACS Version 8.5 Maintenance and Service Guide
Caching Techniques
The cache module supports the following caching techniques to increase subsystem read
and write performance:
■ Read caching
■ Read-ahead caching
■ Write-through caching
■ Write-back caching
Read Caching
When the controller receives a read request from the host, it reads the data from the disk
drives, delivers it to the host, and stores the data in its cache module. Subsequent reads for
the same data will take the data from cache rather than accessing the data from the disks.
This process is called read caching.
Read caching can decrease the subsystem response time to many of the host read requests.
If the host requests some or all of the cached data, the controller satisfies the request from
its cache module rather than from the disk drives. By default, read caching is enabled for
all storage units.
Read-Ahead Caching
Read-ahead caching begins when the controller has already processed a read request, and
it receives a subsequent read request from the host. If the controller does not find the data
in the cache memory, it reads the data from the disks and sends it to the cache memory.
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.