Prefetch Operation - Seagate ST11200N Product Manual

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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 mentioned above.
3. If the prefetch feature is enabled, refer to section 5.5.2 for operation from this point.
Case B - A Read command requests data, the first LB of which is not in any segment of the cache:
1. The drive fetches the requested LB's from the disc and transfers them into a segment, and from there
to the host in accordance with the disconnect/reconnect specification referred to in case A.
2. If the prefetch feature is enabled, refer to section 5.5.2 for operation from this point.
Each buffer segment is actually a self-contained circular storage (wrap-around occurs), the length of which
is an integer number of disc medium sectors. The wrap-around capability of the individual segments greatly
enhances the buffer's overall performance as a cache storage, allowing a wide range of user selectable
configurations, which includes their use in the prefetch operation (if enabled), even when cache operation is
disabled (see section 5.5.2). For the Hawk 1 family the number of segments may be selected using the
Mode Select command, but the size can not be directly selected. Size is selected only as a by-product of
selecting the segment number specification. The size in Kbytes of each segment is reported by the Mode
Sense command page 08h, bytes 14 and 15. If a size specification is sent by the host in a Mode Select
command (bytes 14 and 15) no new segment size is set up by the drive, and if the "STRICT" bit in Mode
page 00h (byte 2, bit 1) is set to one, the drive responds as it does for any attempt to change unchangeable
parameters (see SCSI I/O Product Manual*). The Hawk 1 family of drives supports operation of any integer
number of segments from 1 to 16. Divide the 245,760 bytes in the buffer by the number of segments to get
the size (in bytes) of each segment.
5.5.2

Prefetch operation

If the Prefetch feature is enabled, data in contiguous logical blocks on the disc immediately beyond that
which was requested by a Read command can be retrieved and stored in the buffer for immediate transfer
from the buffer to the host on subsequent Read commands that request those logical blocks (this is true
even if "cache" operation is disabled). Though the prefetch operation uses the buffer as a "cache", finding
the requested data in the buffer is a prefetch "hit", not a "cache" operation "hit". Prefetch is enabled using
Mode Select page 08h, byte 12, bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0 enables prefetch. Since
data that is prefetched replaces data already in some buffer segment(s), the host can limit the amount of
prefetch data to optimize system performance. The drive never prefetches more logical blocks than the
number specified in the prefetch logical blocks ceiling bytes (8 and 9) of Mode page 08h.
During a prefetch operation, the drive crosses a cylinder boundary to fetch more data only if the Discontinu-
ity (DISC) bit is set to one in bit 4 of byte 2 of Mode parameters page 08h.
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*SCSI Interface Product Manual P/N 77738479.
Product Manual - Hawk 1 Family SCSI-2 (Volume 1), Rev. D