Prefetch Operation - Seagate HAWK ST32430N Product Manual

Product Manual - Hawk 2LP Family SCSI-2 (Volume 1), Rev. F 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). 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 drive supports operation of any integer number of segments from 1 to 16.
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 max prefetch field (bytes 8 and 9) limits the amount of prefetch. The drive does not
use the prefetch "ceiling" field (bytes 10 and 11).
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.
Whenever prefetch (read look-ahead) is enabled (enabled by DRA = 0), it operates under the control of ARLA
(Adaptive Read Look-Ahead). ARLA enables prefetch of contiguous blocks from the disk when it senses that
a prefetch "hit" will likely occur, even if two consecutive read operations were not for physically contiguous
blocks of data (e.g. "software interleave"). ARLA disables prefetch when it decides that a prefetch "hit" will not
likely occur. If the host is not using software interleave, and if two sequential read operations are not for
contiguous blocks of data, ARLA disables prefetch, but as long as sequential read operations request con-
tiguous blocks of data, ARLA keeps prefetch enabled.
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*SCSI Interface Product Manual, P/N 77738479.