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For example, if a read operation of 16 blocks with a logical data block length of 512 bytes
(8 Kbytes) is requested, when the value specified in this parameter is 32, reconnection
processing is performed when it becomes possible to transfer 2 blocks of data [16 x
(32/256)], from the data buffer.
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]. 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. 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 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.
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C141-E039-01EN
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