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Data Buffer Management
2 - 6 C141-E167
b. Buffer empty ratio
This parameter specifies the timing for the IDD to start reconnection when it is requested by the
INIT to transfer data in the WRITE, WRITE EXTENDED or WRITE AND VERIFY command.
In a write operation, after the IDD receives a command, the IDD begins transferring data (data
pre-fetch) while locating the position of the target data blocks on the disk media in parallel.
Therefore, if the total number of bytes of data specified in the command is less than the capacity
of a cache segment, all the data are pre-fetched in a batch, so the specification in this parameter
is not applied.
The value specified in this parameter (n) indicates a proportion of the total data buffer capacity
of the IDD when the buffer is empty [n/256]. (Note)
While the IDD is writing data to the disk media, it is using the data pre-fetched to the data buffer
in order and when the empty space in the data buffer reaches the amount specified in this
parameter, it executes reconnection processing, requesting transfer of the succeeding data.
However, if the number of data blocks remaining to be transferred according to the command
being executed does not reach the amount of data specified in this parameter, the IDD executes
reconnection processing at the point when the number of data blocks remaining in the data
buffer's empty space reaches the specified number of data blocks and requests transfer of the
succeeding data.
Note:
If the value specified for the "buffer full ratio" or the "buffer empty ratio" is not within
double the integral boundary of the logical data block length, the IDD rounds up the
specified value to the nearest data block boundary. In addition, if zero is specified for any
of these parameters, the IDD an appropriate value for the parameter that is determind by
IDD.
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 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 2.2.2.