Seagate Elite 23 ST423451N, Руководство по эксплуатации продукта

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Elite 23 Product Manual, Rev. A 11
4.3 Start/stop time
Disabling the Motor Start option causes the drive to become ready within 45 seconds after DC power is applied
at nominal voltage. This means the motor starts as soon as power is applied. If a recoverable error condition is
detected during the start sequence, the drive will execute a recovery procedure and may cause the time to
become ready to exceed 45 seconds. During this time the drive responds to some commands over the SCSI
interface. Stop time is less than 40 seconds from removal of DC power.
Enabling the Motor Start option causes the internal controller to accept the commands listed in the SCSI Inter-
face 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 45 seconds (excluding an error recovery pro-
cedure). The Motor Start command can also be used to command the drive to stop the spindle in less than 40
seconds (see the Start/Stop command information in the
SCSI Interface Product Manual).
There is no power control switch on the drive.
4.4 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 com-
mand with the proper values in the applicable bytes in Mode Page 08h (see
SCSI Interface Product Manual ).
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 Section 4.5.1 and 4.5.2.
All default cache and prefetch Mode parameter values (Mode Page 08h) for standard OEM versions of this
drive family are given in Table 8.
4.5 Cache operation
Of the 2,048 Kbytes physical buffer space, approximately 1,442 Kbytes can be used as a cache. The cache
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 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 = 0 in Mode page 08h, byte 2, bit 0 in the
SCSI 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. 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 opera-
tion 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. On 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 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 - 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.
2. When a requested logical block is reached that is not in any segment, the drive fetches it and any remain-
ing requested logical block addresses from the disc and puts them in a segment of the cache. The drive
transfers the remaining requested logical blocks from the cache to the initiator in accordance with the
“buffer-full” ratio specification given in Mode Select Disconnect/Reconnect parameters, page 02h (see the
SCSI Interface Product Manual).
3. The drive prefetches additional logical blocks contiguous to those transferred in step 2 above and stores
them in the segment. The drive stops filling the segment when the maximum prefetch value has been
transferred (see the
SCSI Interface Product Manual).