Fujitsu 80, Scsi Logical Interface Specifications

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Data Buffer Management
2-4 C141-C010
3) If the data transfer rate of the INIT (SCSI bus) is lower than the data transfer rate of the disk drive, since the
data pre-fetch of data that are to be written to that data block is not completed, it 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) After disconnect processing, 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 reconnection processing and begins transferring the
succeeding data (data pre-fetch).
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, disconnection, reconnection and data transfer 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.
Notes:
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 disconnect and reconnect 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 2.1.2) of the MODE SELECT parameter so the difference in
the data transfer rates between the INIT (SCSI bus) and the disk drive is maintained in a good
balance.
Figure 2.3 shows an example of the data buffer operating state during a write operation. Details are
explained in section 2.1.2.
Amount of Data in the Buffer
SCSI
Bus
Sector
Disk
STATUS
Reconnection Disconnection
DATA OUT
COMMAND
Figure 2.3 Example of data buffer operation during write