Fujitsu MAA3182FC Series, Manual

Page: 135 / 327

C141-E054-02EN 3 - 4
may not be possible to write the data continuously to the disk (data underrun). In this case, the
IDD waits one disk revolution while continuing to pre-fetch data from the INIT, then repositions
the block that caused the data underrun and continues the writing of data to the disk.
4) If all the blocks of data specified in the command have not yet been transferred from the
INIT, as writing of data to the disk media progresses, at the point when the empty space in
the data buffer reaches the amount specified in the buffer empty ratio in the MODE
SELECT parameter, the IDD performs Loop OPEN processing and begins transferring the
succeeding data (data pre-fetch) after it transfers the XFER_RDY frame.
Writing of data to the disk media is performed in parallel and data transfer with the INIT
continues until the empty space in the data buffer is full or until transfer of all the data specified
in the command is completed. From then on, the XFER_RDY data receiving are repeated until
transfer of all the blocks of data specified in the command is completed.
5) When writing of all the specified data is completed, the IDD reports the status and
terminates the command.
IMPORTANT
1) If the amount of transfer data specified in a command is smaller than the
capacity of the cache segment, all the data necessary for execution of the
command are pre-fetched in a batch in the processing in 1) above, so the
value specified in the buffer empty ratio has not meaning.
2) If the amount of data processed with a single command exceeds the
capacity of one cache segment, in order to avoid frequent repetition of
Loop OPEN/CLOSE processing during command execution, and in order
to escape the occurrence of data underrun, it is necessary to set the buffer
empty ratio (see Section 3.1.2) of the MODE SELECT parameter so the
difference in the data transfer rates between the INIT (FC interface) and
the disk drive is maintained in a good balance.
Figure 3.3 shows an example of the data buffer operating state during a write operation.
Details are explained in section 3.1.2.
Figure 3.3 Example of data buffer operation during write