Seagate Hawk 2 ST11900N, Руководство по эксплуатации продукта

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12 Product Manual - Hawk 2 Family SCSI-2 (Volume 1), Rev. C
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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 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 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 Kbytes of 1 MB for drives having the optional 1 MB 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 number of seg-
ments) 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 Mod 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 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 men-
tioned 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.