Compaq HSZ80 manual Write-Through Caching, Write-Back Caching

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The controller then 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 through 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 stores 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.

If you enable read caching for a storage unit, write-through caching is automatically enabled. Likewise, if you disable read caching, write-through caching is automatically disabled.

Write-Back Caching

This caching technique decreases 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 disk units. In either case, the controller will not provide write-back caching to a unit unless the cache memory is nonvolatile, as described in the next section.

If the mirrorset is a disaster-tolerant mirrorset, then write-back caching cannot be enabled.

Compaq HSZ80 Array Controller ACS Version 8.3 Configuration and CLI Reference Guide