Seagate ST12450W, Руководство по эксплуатации продукта

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16 ST12450W/WD Product Manual, Rev. A
2. When a requested logical block is reached that is not in any segment,
the drive fetches it and any remaining 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 Sel ect Disconnect/Reconnect parameters, page 02h (see the
SCSI-2 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-2 Interface Product Manual).
Case B. A read command is received and the first logical block address
requested is not in any segment of the cache.
1. The drive fetches the requested logical blocks from the disc and
transfers them into a segment, then from there to the initiator in
accordance with the “buffer-full” ratio specification given in Mode Select
Disconnect/Reconnect parameters, page 02h (see the
SCSI-2 Inter-
face Product Manual).
2. The drive prefetches additional logical blocks contiguous to those trans-
ferred in Case A, step 2 above and stores them in the segment. The drive
stops filling the segment when the maximum prefetch value has been
transferred.
During a prefetch, the drive crosses a cylinder boundary to fetch data only
if the Discontinuity (DISC) bit is set to 1 in bit 4 of byte 2 of the Mode Select
parameters page 8h. Default is zero for bit 4 (see the
SCSI-2 Interface
Product Manual).
Each cache segment is actually a self-contained circular buffer whose
length is an integer number of sectors. The wrap-around capability of the
individual segments greatly enhances the cache’s overall performance,
allowing a wide range of user-selectable configurations including a pure
prefetch strategy. The drive supports operation of any integer number of
segments from 1 to 16. Divide the 976,896 bytes in the buffer by the number
of segments to get the segment size. Default is 3 segments. (See the
SCSI-
2 Interface Product Manual.)
5.5.1 Adaptive read lookahead
Read lookahead causes the drive to continue reading data from the disc
following a normal read I/O until the read-ahead parameters are satisfied
(prefetch). If subsequent I/O requests can be satisfied from the prefetched
data in the data buffer, there is a significant improvement in performance
since a disc access is eliminated. If subsequent I/O requests cannot be
satisfied from the prefetched data, there is a reduction in performance due
to prefetch overhead.
The adaptive read lookahead feature suspends the prefetch operation if
three subsequent read I/O requests are not satisfied from the prefetched
data. This improves performance because unnecessary prefetches are
avoided.
Prefetch is reinstated if an I/O request is sequential to a previous I/O request.