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a. Buffer Full Ratio
This parameter specifies the timing for the IDD to start reconnection processing in order to
transfer data to the INIT by the READ or READ EXTENDED command.
The value specified in this parameter (n) shows the amount of data read to the IDD's data
buffer from the disk media as a proportion [n/256] of the total capacity of the data buffer
until reconnection processing with the SCSI bus is started. The IDD's default segment
capacity is 127 Kbytes. Therefore, if the specified value (n) is 128, for example, it means
that a data volume of 63.5 Kbytes was specified (Note)
If it is possible to transfer the amount of data specified in this parameter to the INIT from
the data buffer, reconnection processing is executed and transfer of data to the INIT
begins.
The default value for this parameter is X '20.' It is possible to specify X '00' in this
parameter, but the minimum effective value of the buffer full ratio is 1 logical data block,
so specifying X '00' is equivalent to specifying the value for 1 logical data block. (Note)
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.
The value specified in this parameter (n) indicates the size of the empty area in the IDD's
data buffer as a proportion of the total data buffer capacity of the IDD [n/256]. For
example, if the specified value (n) is 128 and the number of cache segments is 4 (this is the
default value), the size of the empty space in the buffer is specified as 63.5 Kbytes. (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.
If the total number of bytes in the transferred data block count specified in the command is
less than the capacity of the buffer segment, all the data are pre-fetched in a batch, so the
specification in this parameter is not applied.
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 value which is the nearest data block boundary. Also, if
zero is specified in any of these parameters, the IDD treats that parameter as though 1
logical data block has been specified. However, parameter values specified from the
INIT in the MODE SELECT or MODE SELECT EXTENDED command are held as
is and those values are reported in response to a MODE SENSE or MODE SENSE
EXTENDED command. (Rounding processing of the parameters is not done by the
IDD.) Also, if the specified value does not reach 1 block, it is regarded as having
specified 1 block.
C141-E039-01EN
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