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4) After disconnect processing, if all the blocks of data specified in the command have not
yet been transferred by 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 item 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.
Fig. 2.3 shows an example of the data buffer operating state during a write operation.
Details are explained in item 2.1.2.
Disk
Amount of Data in the Buffer
SCSI
Bus
COMMAND
Figure 2.3
2 - 4
Sector
DATA OUT
Disconnection
Example of Data Buffer Operation during Write
C141-E039-01EN
STATUS
Reconnection
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