Product manual

12 Product Manual - Hawk 2 Family (Wide bus) SCSI-2 (Volume 1), Rev. A
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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 15 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 (984 KB of 1 MB for drives having 1 MB optional buffer). 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 num-
ber 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 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 already
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 remain-
ing 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 speci-
fication 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.