Acer SmartRAID V Bedienungsanleitung Seite 180

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Seitenansicht 179
7-12
Theory of Operation
Caching Host Reads
Because there is no advantage in using controller cache to duplicate the
operating system cache, SmartRAID V controllers with cache only cache
predictive and dirty data. Read cache misses are transferred directly from
disk to host, bypassing the controller cache.
Optimizing Operating System Cache Size
When configuring a server, you should always consider the effect of the
operating system cache on the disk subsystem. Although many disk read
operations can be eliminated by the operating system cache, all writes
must get through to the disk. In effect, the operating system cache is
acting as a read filter for the disk subsystem.
As the operating system cache size is increased, more read hits are
serviced from the operating system cache resulting in fewer reads issued
to the disk subsystem. However, the number of disk writes stays relatively
constant. At some point, adding more cache to the operating system will
not result in significantly reduced disk I/O. At this point it is better to add
more cache to the controller to improve write throughput.
NOTE Most operating systems that have caching capability can
automatically manage the size of their disk cache.
Optimizing Controller Cache Size
The SmartRAID V cache is used for predictive reads, elevator sorted
write-backs and as temporary storage during RAID 5 parity calculations.
Increasing the size of the cache makes these features more efficient. More
cache RAM allows the controller to hold and elevator-sort more records,
resulting in closer spacing between consecutive write-back segments. This
results in lower average access time for disk write operations and higher
disk throughput.
The SmartRAID V cache can be expanded incrementally as needed to
maintain optimum system performance. More cache is typically required
for systems with many active users than for single-user systems. Systems
with large data files also benefit from a larger cache. However, a system
that has a large number of users who all access the same data, will require
less cache than the previous examples.
NOTE DPT recommends a minimum of 16MB of cache for
SmartRAID V controllers.
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