Seagate Cheetah 15K.4 SAS, Руководство по эксплуатации продукта

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Cheetah 15K.4 SAS Product Manual, Rev. B 15
During a prefetch operation, the drive crosses a cylinder boundary to fetch more data only if Mode parameters
page 08h, byte 2, bit 4 is set to 1 (Discontinuity--DISC bit).
When prefetch (read look-ahead) is enabled (enabled by DRA = 0), it operates under the control of ARLA
(Adaptive Read Look-Ahead). If the host uses software interleave, ARLA enables prefetch of contiguous
blocks from the disc when it senses that a prefetch hit will likely occur, even if two consecutive read operations
were not for physically contiguous blocks of data (e.g. “software interleave”). ARLA disables prefetch when it
decides that a prefetch hit will not likely occur. If the host is not using software interleave, and if two sequential
read operations are not for contiguous blocks of data, ARLA disables prefetch, but as long as sequential read
operations request contiguous blocks of data, ARLA keeps prefetch enabled.
4.5.3 Optimizing cache performance for desktop and server applications
Desktop and server applications require different drive caching operations for optimal performance. This
means it is difficult to provide a single configuration that meets both of these needs. In a desktop environment,
you want to configure the cache to respond quickly to repetitive accesses of multiple small segments of data
without taking the time to “look ahead” to the next contiguous segments of data. In a server environment, you
want to configure the cache to provide large volumes of sequential data in a non-repetitive manner. In this
case, the ability of the cache to “look ahead” to the next contiguous segments of sequential data is a good
thing.
The Performance Mode (PM) bit controls the way the drive switches the cache buffer into different modes of
segmentation. In “server mode” (PM bit = 0), the drive can dynamically change the number of cache buffer
segments as needed to optimize the performance, based on the command stream from the host. In “desktop
mode” (PM bit = 1), the number of segments is maintained at the value defined in Mode Page 8, Byte 13, at all
times (unless changed by using a Mode Select command). For additional information about the PM bit, refer to
the Unit Attention Parameters page (00h) of the Mode Sense command (1Ah) in the SAS Interface Manual,
part number 100293071.