Seagate ST15150N Product Manual - Page 26

SCSI-2 Interface, Product Manual., SCSI-2 Interface Product Manual, SCSI-2 Interface Product, Manual

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16 ST15150N/ND/W/WD/WC/DC Product Manual, Rev. D 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 Select 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. 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 Interface Product Manual). 2. The drive prefetches additional logical blocks contiguous to those transferred 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, which includes 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.)

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16
ST15150N/ND/W/WD/WC/DC Product Manual, Rev. D
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 Select
Disconnect/Reconnect parameters, page 02h (see the
SCSI-2 Interface
Product Manual).
3.
The drive prefetches additional logical blocks contiguous to those trans-
ferred in step 2 above and stores them in the segment. The drive stops
filling the segment when the maximum prefetch value has been trans-
ferred (see the
SCSI-2 Interface Product Manual)
.
Case B.
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 Interface 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, which includes 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.)