Staggered Spinup; Activity Indication; Cacheflow; Write Cache - Western Digital Purple PR1200M Technical Reference Manual

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WD Purple™ PR1200M

also be used by the device to provide the host with an activity indication. According

to the SATA spec, "Staggered Spin-up Disable and Activity Signal shall not be enabled

at the same time." Note: This feature may not be available in all configurations.

3.19.1

When multiple disks are installed in an enclosure, it is desirable to provide a simple

mechanism by which a subsystem controller can sequence hard drive initialization to

minimize the current load presented during power up. Staggered spinup provides

this mechanism by preventing the hard drives from spinning up until after successful

PHY initialization (i.e., after PHY enters DP7:DR_Ready state).

Staggered spinup is only applicable during initial power-up. If a drive is spun down

using ATA commands—as a result of having been placed in Standby or Sleep power

modes, for example—the drive shall spin up following the rules that govern spinup

from low power modes described in ATA/ATAPI-6 or later.

3.19.2 Activity Indication

The host controller through SATA power pin 11 may access storage device status and

activity. The signal provided by the device for activity indication is a low-voltage low-

current signal. It is not suitable for directly driving an LED. A buffer circuit external to

the device must be employed to drive the LED. The activity signal is based on an

open-collector or open-drain active low driver. The device shall tolerate the activity

signal being shorted to ground.

3.20 CacheFlow™

CacheFlow is WD's unique, multi-generation disk caching system. It incorporates read

cache with write cache.

WD designed CacheFlow to obtain maximum performance with today's most popular

operating systems and applications. CacheFlow increases performance over prior

caching algorithms by increasing the number of times that requested data is in the

cache. This reduces the number of host commands that require actual media access

thereby improving overall drive performance.

Typical applications perform a variety of access patterns, such as random, sequential,

and repetitive. CacheFlow is designed to dynamically adapt to the changes in access

patterns that occur during the course of application execution.

Random mode is the default operational mode for CacheFlow. Once CacheFlow

detects a sequential access pattern, it leaves random mode. CacheFlow also

performs predictive read operations to increase the probability that data requested in

future commands already exists in the cache.

CacheFlow partitions the buffer into multiple segments to allow for the fact that

applications may access multiple non-contiguous areas on the disk. CacheFlow tracks

the amount of valid data in each segment and controls the deallocation of segments

to maximize drive performance.

3.20.1 Write Cache

CacheFlow is designed to improve both single and multi-sector write performance by

reducing delays caused by seek time and rotational latency.

The write cache adaptively detects random and sequential access patterns during

application execution.

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