Acer SmartRAID V Bedienungsanleitung PDF herunterladen (Seite 175)

7-7

Theory of Operation

Software and Hardware Caching

The SmartRAID V cache is designed to enhance performance in

environments that are already software cached, such as UNIX, Novell and

Windows NT. Although software caches are beneficial, hardware caching

offers a significant advantage in larger, heavily loaded systems when

combined with the software cache for the operating system.

The purpose of the operating system cache is to minimize disk reads. If

data requested by an application is found in the operating system cache,

a disk read is eliminated. A software cache can also improve application

response time by immediately accepting data to be written to disk from

application programs and postponing the actual writes until the disk is

idle.

LIGHTLY LOADED SYSTEM

Without Hardware Cache

Application programs write to OS cache:

Bus

activity:

OS cache flushes to disk (~11ms per write)

Idle

time

Idle

time

Although write caching through the operating system can benefit a lightly

loaded system, it can result in performance degradation in large multi-

user environments. The greater I/O demands in such systems reduce the

periods of system idle time required to flush the operating system cache to

disk. As disk activity becomes more intense, the periods of system idle

time become shorter and less frequent. The operating system cache fills

up with dirty sectors waiting to be written to disk, which take up space that

could be used for more recently requested data. When cache RAM is

added to a SmartRAID V controller, the operating system cache buffers

must still be flushed. However, the controller receives and caches the

flushed data in a fraction of the time required to actually write to a disk.

The controller can also write the data back to disk in an elevator-sorted

order without interrupting other system activity.

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Acer SmartRAID V Bedienungsanleitung Seite 175