Start/Stop Time; Prefetch/Multi Segmented Cache Control; Cache Operation - Seagate ST11200N Product Manual

Product Manual - Hawk 1 Family SCSI-2 (Volume 1), Rev. D
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5.4

Start/stop time

After DC power has been applied, the drive becomes ready within 20 seconds (17 seconds typical) if the
Motor Start Option is disabled (i.e. the motor starts as soon as the power has been applied). During this time
the drive will transfer commands over the SCSI interface but may take up to 3 seconds to respond back to
the host. Stop time is less than 20 seconds from removal of DC power.
If the Motor Start Option is enabled the internal controller accepts the commands listed in the SCSI Interface
Product Manual* less than 3 seconds after DC power has been applied. After the Motor Start Command has
been received the drive typically becomes ready for normal operations within 13.5 seconds. The Motor Start
Command can also be used to command the drive to stop the spindle*.
There is no power control switch on the drive.
5.5 Prefetch/multi-segmented cache control
The drive provides prefetch (read look-ahead) and multi-segmented cache control algorithms that in many
cases can enhance system performance. "Cache" as used herein refers to the drive buffer storage space
when it is used in "cache" operations. To select prefetch and cache features the host sends the Mode Select
command with the proper values in the applicable bytes in Mode Page 08h*. Prefetch and cache operation
are independent features from the standpoint that each is enabled and disabled independently via the Mode
Select command. However, in actual operation the prefetch feature overlaps cache operation somewhat as
is noted in sections 5.5.1 and 5.5.2.
All default cache and prefetch Mode parameter values (Mode Page 08h) for standard OEM versions of this
drive family are given in Tables 11.3.2-1 through 11.3.2-4.
5.5.1

Cache operation

In general, 240 Kbytes of the 256 Kbytes of physical buffer space in the drive can be used as storage space
for cache operations. The buffer can be divided into logical segments (Mode Select Page 08h, byte 13) from
which data is read and to which data is written. The drive maintains a table of logical block disk medium
addresses of the data stored in each segment of the buffer. If cache operation is enabled (RCD bit = 0 in
Mode Page 08h, byte 2, bit 0. See SCSI Interface Product Manual*), data requested by the host with a Read
command is retrieved from the buffer (if it is there), before any disc access is initiated. If cache operation is
not enabled, the buffer (still segmented with required number of segments) is still used, but only as circular
buffer segments during disc medium read operations (disregarding Prefetch operation for the moment).
That is, the drive does not check in the buffer segments for the requested read data, but goes directly to the
medium to retrieve it. The retrieved data merely passes through some buffer segment on the way to the
host. All data transfers to the host are in accordance with "buffer-full" ratio rules. See explanations associ-
ated with Mod page 02h (disconnect/reconnect control) in the SCSI Interface Product Manual*.
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*SCSI Interface Product Manual, P/N 77738479.
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