Start/Stop Time; Prefetch/Multi-Segmented Cache Control - Seagate ST15150N - Barracuda 4.3 GB Hard Drive Product Manual

ST15150N/ND/W/WD/WC/DC Product Manual, Rev. D
5.4
5.5

Start/stop time

Disabling the Motor Start option causes the drive to become ready within 30
seconds after DC power is applied. If a recoverable error condition is
detected during the start sequence, the drive executes a recovery proce-
dure this may cause the time it takes for the drive to become ready to exceed
30 seconds. During the start sequence, the drive responds to some com-
mands over the SCSI interface. Stop time is less than 30 seconds after
removing DC power. 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 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 seconds
(excluding an error recovery procedure, if needed). The Motor Start com-
mand 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.

Prefetch/multi-segmented cache control

The drive provides a prefetch/multi-segmented cache algorithm that 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 954 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, if possible, 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, 954
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. Read command is received and the first logical block is already in
the cache.
1. 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.
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