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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 IDD 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 block groups on the disk media sequentially using multiple commands.
3.2.1
Caching operation
When the IDD 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 IDD transfers those data directly to
the INIT from the data buffer without accessing the disk media. In this way, when continuous
logical data block groups 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 3.2.2.
(1)
Caching object commands
Commands which execute caching operations are the commands shown below:
•
READ
•
READ EXTENDED
If all the data block groups 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
IDD receives a command, it immediately opens the loop and 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 IDD 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 oldest cache segment (selected by time stamp).
(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.
a) Data read by a READ or a READ EXTENDED command and which have been read to the
data buffer by Look-Ahead are data which are objects of caching. Depending on the
timing for halting Look-Ahead, it is possible that data read by the READ command will be
overwritten by Look-Ahead.
C141-E054-02EN
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