C141-E039-01EN
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2.2
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.
2.2.1
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 final 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 item 2.2.2.
(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 and performs disconnect processing, it immediately reconnects 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.
