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11
Each cache segment is actually a self-contained circular buffer whose length is an integer number of logical blocks. The
drive dynamically creates and removes segments based on the workload. The wrap-around capability of the individual
segments greatly enhances the cache’s overall performance.
Note.
The size of each segment is not reported by Mode Sense command page 08h, bytes 14 and 15. The value
0XFFFF is always reported regardless of the actual size of the segment. Sending a size specification using the
Mode Select command (bytes 14 and 15) does not set up a new segment size. 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 an unchangeable param-
eter.
4.5.1
Caching write data
Write caching is a write operation by the drive that makes use of a drive buffer storage area where the data to be written to
the medium is stored while the drive performs the Write command.
If read caching is enabled (RCD=0), then data written to the medium is retained in the cache to be made available for future
read cache hits. The same buffer space and segmentation is used as set up for read functions. The buffer segmentation
scheme is set up or changed independently, having nothing to do with the state of RCD. When a write command is issued, if
RCD=0, 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 exceed 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.
If write caching is enabled (WCE=1), then the drive may return Good status on a write command after the data has been
transferred into the cache, but before the data has been written to the medium. If an error occurs while writing the data to the
medium, and Good status has already been returned, a deferred error will be generated.
The Synchronize Cache command may be used to force the drive to write all cached write data to the medium. Upon
completion of a Synchronize Cache command, all data received from previous write commands will have been written to the
medium. Section 11.3.2 shows the mode default settings for the drive.
4.5.2
Prefetch operation
If the Prefetch feature is enabled, data in contiguous logical blocks on the disk immediately beyond that which was requested
by a Read command are 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.
To enable Prefetch, use Mode Select page 08h, byte 12, bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0 enables prefetch.
The drive does not use the Max Prefetch field (bytes 8 and 9) or the Prefetch Ceiling field (bytes 10 and 11).
When prefetch (read look-ahead) is enabled (enabled by DRA = 0), the drive enables prefetch of contiguous blocks from the
disk when it senses that a prefetch hit will likely occur. The drive disables prefetch when it decides that a prefetch hit is not
likely to occur.
