Fujitsu MAA3182SC Oem Manual page 60

Note:
In order to avoid frequent repetition of the disconnect/reconnect processing after data
transfer starts on the SCSI bus, and escape the occurrence of data overruns, it is
necessary to set the buffer full 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. 2.2 shows an example of the data buffer operating state during a read operation. Details
are explained in item 2.1.2.
Disk
Amount of Data
in the Buffer
SCSI Bus
Data Storage Completion Wait
Figure 2.2
(2) Write Operation
After data transferred from the INIT are stored temporarily in the data buffer, they are written
to the disk media. The basic write operation procedure is as shown below.
1) When the IDD receives a command and disconnects, it immediately reconnects and enters
the DATA OUT phase and stores data transferred from the INIT to the data buffer (data
pre-fetch). At this time, the IDD locates the position of the data block on the disk media
specified by the command in parallel.
When storing of all the data specified in the command in the data buffer is completed, or if the
empty space in the data buffer is all filled during data storage due to the volume of the specified
data, the IDD performs disconnect processing and disconnects from the SCSI bus.
2) When locating of the target data block on the disk media is completed, the IDD writes the
data in the data buffer to the disk media. If the target data block location operation is
completed during the data pre-fetch processing in 1), writing of data to the disk media is
performed in parallel with the data pre-fetch to the data buffer.
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.
Sector
Buffer Full Ratio
Reconnection
Example of Data Buffer Operation during Read
C141-E039-01EN
DATA IN
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