HP StorageWorks HSG60 and HSG80 Array Controller and Array Controller Software Troubleshooting Guide (EK-G80TS-SA. C01, March 2005)

Troubleshooting Information
44 HSG60 and HSG80 Array Controller and Array Controller Software Troubleshooting 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
■ Writeback caching
Read caching
If the controller receives a read request from the host, the controller reads the data
from the disk drives, delivers the data to the host, and stores the data in the
supporting cache module. Subsequent reads for the same data take this data from
the supporting cache module rather than access the data from the disk drives. This
process is called read caching.
Read caching can decrease the subsystem response time to many host read
requests. If the host requests some or all of the cached data, the controller satisfies
the request from the supporting cache module rather than from the disk drives.
Read caching is enabled by default for all storage units.
For more details, refer to the following CLI commands in the controller CLI
reference guide:
■ SET unit-number MAXIMUM_CACHED_TRANSFER=nn
■ SET unit-number MAX_READ_CACHED_TRANSFER_SIZE=nn
■ SET unit-number READ_CACHE
Read-ahead caching
Read-ahead caching begins after the controller has already processed a read
request and the controller receives a subsequent read request from the host. If the
controller does not find the data in the cache memory, the controller reads the data
from the disk drives and sends this data 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 disk drives as the controller
sends the requested read data to the host. These are parallel actions. The controller
notifies the host of the read completion, and subsequent sequential read requests
are satisfied from the cache memory. Read-ahead caching is enabled by default
for all disk units.