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Data Buffer Management
2 - 2 C141-E167
Segment 7
Segment 6
Segment 5
Segment 4
←
Read
←Read
Write→
Segment 3
Segment 2
Segment 1
Segment 0
Valid Data
Write→
Disk Media
INIT
Segment
Figure 2.1 Data buffer configuration (in the case of 8 cache segments)
The basic functions and operations of the data buffer in a read operation and a write operation are
shown below.
(1) Read operation
Data read from the disk media are temporarily stored in the data buffer. Then, they are transferred to
the SCSI bus with the timing specified in the MODE SELECT parameter. The basic read operation
procedure is as shown below.
1) When the IDD receives a command, normally, it disconnects from the SCSI bus through
disconnect processing and locates the data block specified by the command where it is stored on
the disk media.
2) When the target data block has been located, the IDD reads the data form the disk media to the
data buffer. At this time, if a recoverable error is detected, the IDD performs error recovery
processing according to the specification in the MODE SELECT parameter or corrects the data
in the data buffer.
3) When the IDD has read the amount of data specified by the buffer full ratio in the MODE
SELECT parameter (see Section 2.1.2) to the data buffer, it performs reconnection processing
with the SCSI bus and begins transferring data to the INIT (SCSI bus) from the data buffer while
continuing to read subsequent data blocks to the data buffer.
4) If the data transfer capacity of the INIT (SCSI bus) is high and the data buffer becomes empty
before completing transfer of the number of data blocks specified by the command, the IDD
performs disconnect processing at that point. Thereafter, reconnection, data transfer and
disconnection (the operations in 3) and 4) above) are repeated until all the data blocks specified
in the command have been transferred.
5) If the number of blocks specified in the command to be transferred is larger than the capacity of
a cache segment and if the data transfer rate of the INIT is lower than the data transfer rate of the
disk drive, the empty space in the data buffer disappears from step 3) and the IDD may soon not
be able to read data from the disk media (data overrun). In this case, the IDD relocates the block
that caused the data overrun after waiting one disk revolution, and continues the reading process.