Prefetch Operation - Seagate ST34501N Product Manual

12 Cheetah 4LP Product Manual, Rev. B
ing nothing to do with whether or not read and write caching is enabled or disabled. When a write command is
issued, the cache is first checked to see if any logical blocks that are to be written are already stored in the
cache from a previous read or write command. If there are, the respective cache segments are cleared. The
new data is cached for subsequent Read commands.
If the number of write data logical blocks exceeds the size of the segment being written into when the end of
the segment is reached, the data is written into the beginning of the same cache segment, overwriting the data
that was written there at the beginning of the operation. However, the drive does not overwrite data that has not
yet been written to the medium.
Table 9 shows Mode default settings for the drive.
4.5.2

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). If the host uses software interleave, ARLA enables prefetch of contiguous
blocks from the disc when it senses that a prefetch "hit" will likely occur, even if two consecutive read opera-
tions 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 contiguous blocks of data, ARLA keeps prefetch enabled.