Read Look-Ahead And Caching; Write Caching And Write Merging - Seagate MEDALIST 1080SL Product Manual

Medalist 1080sl SCSI Product Manual, August 1995
C.6.1 Read look-ahead and caching
The drive uses an algorithm that improves seek performance by reading
the next logical sectors after the last requested sector. These unre-
quested sectors are read into a buffer and are ready to be transmitted to
the host before they are requested. Because these sectors are read
before they are requested, access read time for the sectors is virtually
eliminated. This process is called either read look-ahead or read caching.
Read look-ahead and read caching are similar algorithms. Read look-
ahead occurs when a Read command requests more data than can be
contained in one buffer segment. Read caching occurs when a Read
command requests less data than can be contained in one buffer
segment.
The buffer used for read look-ahead and caching can be divided into
segments as shown in the following table. To change the number of
segments, use byte 13 of the Caching page, which is described in
Appendix C.6.3. The default is one 128-Kbyte segment.
Number of segments
1
2
4
When the buffer is divided into multiple segments, each segment func-
tions as an independent buffer, causing dramatically increased perform-
ance in multitasking and multiuser environments.
C.6.2 Write caching and write merging
Write caching. The drive uses the write segment to store Write com-
mands and data. After the drive caches the commands and data, it is
immediately ready to process new commands. The drive writes the data
to the disc at its next convenient opportunity.
Write merging. The drive accepts contiguous Write commands and
executes them sequentially as one command.
Size of segment (in Kbytes)
128
64
32
89