Seagate ST12450W, Руководство по эксплуатации продукта

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ST12450W/WD Product Manual, Rev. A 15
5.4 Start/stop time
Disabling the Motor Start option causes the drive to become ready within 30
seconds after DC power is applied at nominal voltage. This means the motor
starts as soon as power is applied. During this time the drive responds to
some commands over the SCSI interface. Stop time is less than 30 seconds
(maximum) from removal of DC power.
Enabling the Motor Start option causes the internal controller to accept the
commands listed in the
SCSI-2 Interface Product Manual (77738479) less
than 3 seconds after applying DC power. After receiving the Motor Start
command, the drive becomes ready for normal operations within 30 sec-
onds. The Motor Start command can also be used to command the drive to
stop the spindle (see the Start/Stop command information in the
SCSI-2
Interface Product Manual).
There is no power control switch on the drive.
5.5 Prefetch/multi-segmented cache control
The drive provides a prefetch/multi-segmented cache algorithm, which in
many cases enhances system performance. To select this feature, the host
sends the Mode Select command with the proper values in the applicable
bytes in page 08h (see the
SCSI-2 Interface Product Manual). Default is
prefetch and cache operation enabled.
Of the 1,024 Kbytes physical buffer space, approximately 960 Kbytes can be
used as a cache. The cache can be divided into logical segments from which
data is read and to which data is written.
The drive keeps track of the logical block addresses of the data stored in each
segment of the cache. If the cache is enabled (see RCD bit, Table 5.2.1-27
in the
SCSI-2 Interface Product Manual), data requested by the host with a
read command is retrieved from the cache before any disc access is initiated.
Data in contiguous logical blocks immediately beyond that requested by the
Read command can be retrieved and stored in the cache for immediate
transfer to the initiator on subsequent read commands. This is referred to as
the prefetch operation. Since data that is prefetched may replace data
already in the cache segment, an initiator can limit the amount of prefetch
data to optimize system performance. The drive never prefetches more
sectors than the number specified in bytes 8 and 9 of Mode page 08h (see
the
SCSI-2 Interface Product Manual). If the cache is not enabled, 960
Kbytes of the buffer are used as a circular buffer for read/writes, with no
prefetch operation and no segmented cache operation.
The following is a simplified description of the prefetch/cache operation:
Case A. A read command is received and the first logical block is already in
the cache.
1. The drive transfers to the initiator the first logical block requested plus all
subsequent contiguous logical blocks that are already in the cache. This
data may be in multiple segments.