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5.4
Start/stop time
After DC power has been applied, the drive becomes ready within 25 seconds 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 responds to
some commands over the SCSI interface* in less than 3 seconds. Stop time is 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 becomes ready for normal operations within 25 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 and 11.3.2-2.
5.5.1
Cache operation
In general, 448 Kbytes of the 512 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.
For a cache miss, all data transfers to the host are in accordance with "buffer-full" ratio rules. On a cache hit
the drive ignores the buffer full ratio. See explanations associated with Mode page 02h (disconnect/reconnect
control) in the SCSI Interface Product Manual*.
The following is a simplified description of a read operation with cache operation enabled:
Case A - A Read command is received and the first logical block (LB) is already in cache:
1.
Drive transfers to the initiator the first LB requested plus all subsequent contiguous LB's that are al-
ready in the cache. This data may be in multiple segments.
2.
When the requested LB is reached that is not in any cache segment, the drive fetches it and any
remaining requested LB's from the disc and puts them in a segment of the cache. The drive transfers
the remaining requested LB's from the cache to the host in accordance with the disconnect/reconnect
specification mentioned above.
3.
If the prefetch feature is enabled, refer to section 5.5.2 for operation from this point.
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*SCSI Interface Product Manual P/N 77738479.
Product Manual - Hawk 4 Family (Wide) SCSI-2 (Volume 1), Rev. E
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