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Data Buffer Management
2 - 4 C141-E167
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
ReconnectionDisconnection
DATA OUT
COMMAND
Figure 2.3 Example of data buffer operation during write