Fujitsu MAX3073NC - SCSI,80, Dell DC961,Hard Drive Scsi Logical Interface Specifications page 56

Data Buffer Management
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.
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