Prefetch Operation - Seagate ST12400N Product Manual

Product Manual - Hawk 2 Family SCSI-2 (Volume 1), Rev. B 13
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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 2 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 2
family of drives supports operation of any integer number of segments from 1 to 16. Divide the 245,760 bytes
(1,007,616 bytes for the optional 1 MB buffer) 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 perfor-
mance. 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 Discontinuity
(DISC) bit is set to one in bit 4 of byte 2 of Mode parameters page 08h.
The operation of the prefetch feature (when enabled by DRA = 0) can be modified to an adaptable prefetch mode,
wherein prefetch is disabled as long as two sequential read operations are not for contiguous blocks of data. If
two sequential Read operations request contiguous logical blocks of data, the drive automatically enables the
prefetch feature for the end of the second Read operation. As long as sequential Read operations request con-
tiguous LB's of data, prefetch operation remains enabled.
If the host uses software interleave, there will not likely ever be two sequential Read operations for contiguous
LB's, so the adaptive prefetch feature keeps prefetch disabled, even if the DRA bit enables it. Adaptive prefetch
is enabled when the IC (Initiator Control) bit (Mode page 08h, byte 2, bit 7) is set to 0 and when DRA. = 0 (prefetch
enabled).
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*SCSI Interface Product Manual, P/N 77738479.