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

Troubleshooting Information
45HSG60 and HSG80 Array Controller and Array Controller Software Troubleshooting Guide
Write-through caching
After the controller receives a write request from the host, the controller places the
data in the supporting cache module, writes the data to the disk drives, and then
notifies the host that 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 along the way to the disk drives.
If read caching is enabled for a storage unit, write-through caching is
automatically enabled.
Writeback caching
Writeback caching improves the subsystem response time to write requests by
allowing the controller to declare the write operation complete as soon as the data
reaches the supporting cache memory. The controller performs the slower
operation of writing the data to the disk drives at a later time. 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_WRITE_CACHED_TRANSFER_SIZE=nn
■ SET unit-number WRITEBACK_CACHE
Writeback caching is enabled by default for all units. The controller only provides
writeback caching to a unit if the cache memory is nonvolatile, as described in the
next section.
By default, the controller expects to use an ECB as the backup power source for
the cache module. However, if the subsystem is protected by a UPS, use one of
the following CLI commands to instruct the controller to use the UPS:
SET controller UPS=NODE_ONLY
or
SET controller UPS=DATACENTER_WIDE
Fault-tolerance for writeback caching
The cache module supports nonvolatile memory and dynamic cache policies to
protect the availability of cache module unwritten (writeback) data.