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Data Buffer Management
130 C141-C013
3.2 Look-Ahead Cache Feature
In order to use the data buffer more effectively and improve the disk drive's
effective access speed, the HDD is equipped with a simple cache feature called a
"Look-Ahead Cache Feature." The Look-Ahead cache feature is an effective,
simple cache function for an INIT which reads data blocks on the disk media
sequentially using multiple commands.
3.2.1 Caching operation
When the HDD is executing a READ command or a READ EXTENDED command,
it reads the requested data from the disk media and transfers it to the INIT while at the
same time reading (looking ahead at) data blocks which are subsequent to the last
logical data block specified in that command to the data buffer. Then, when a READ
command or READ EXTENDED command issued later specifies any of those Look-
Ahead data blocks, the HDD transfers those data directly to the INIT from the data
buffer without accessing the disk media. In this way, when continuous logical data
blocks are read sequentially by multiple commands, mechanical access operations can
be eliminated and effective access time greatly reduced.
An INIT can prohibit the operation of this Look-Ahead cache feature by setting
the caching parameter described in Section 5.1.6.
(1) Caching object commands
Commands which execute caching operations are the commands shown below:
• READ
• READ EXTENDED
If all the data blocks which are objects of the processing specified in these
commands, or a portion of the data blocks, including the top logical data block
specified in these commands are relevant data (See item (2)) for caching in the
data buffer (if they hit), when the HDD receives a command, it immediately sends
the data on the data buffer which can be transferred to the INIT. The caching
operation can be disabled by the FUA bit of the READ EXTENDED command.
If "1" is specified in the FUA bit, the HDD reads data from the disk media.
On the other hand, if the top logical data block specified in a command is not
relevant data for caching in the data buffer (if they miss), reading of data is
performed from the disk media using the old cache segment.
(2) Data which are objects of caching
Data which exist in the data buffer and which are objects of caching in item (1)
(data which are not accessed from the disk media but are transferred to the INIT
from the data buffer) are as described below.