Fujitsu 80, Scsi Logical Interface Specifications

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C141-C010 2-1
CHAPTER 2
Data Buffer Management
2.1 Data Buffer
2.2 Look-Ahead Cache Feature
2.3 Write Cache
In this chapter, the configuration of the data buffer with which the IDD is equipped, its operation and the
operation of the cache feature are described.
2.1 Data
Buffer
2.1.1 Data buffer configuration and basic operation
The IDD is equipped with a data buffer, which makes it possible to efficiently execute data transfer
operations between INIT (initiator) and a disk drive.
The IDD divides data buffer into 64 Pages, which are minimum allocation size for Cache. Actually two
or more Pages are linked and a Cache Segment is constructed per the requested data size from INIT.
Since IDD automatically controls the best suitable Cache Segment, INIT cannot be changed neither the
Page size nor the Segment size.
Each data buffer is 2 ports of FIFO (First-In First-Out) ring buffer, with one port allocated for transfers
with the disk media and the other port for data transfers with the SCSI bus. Mutual ports operate
asynchronously and since it is possible to efficiently absorb variations in data transfer speed between the
ports, the INIT can perform data transfer operations with the IDD while being virtually unaware of
differences between the SCSI bus data transfer rate and the disk drive data transfer rate.
Even if the data transfer capacity of the SCSI bus (INIT) is lower than the disk drive's data transfer rate,
the data buffer can perform data transfers to the system under optimum conditions without using sector
interleave. Also, if the data transfer capacity of the SCSI bus (INIT) is higher than the disk drive's data
transfer rate, it is possible to minimize the occupancy time on the SCSI bus by accumulating an
appropriate amount of data in the data buffer.
Only one cache segment by uniting two or more pages of the divided data buffer is used by one
command and the data in the other cache segments are held.
Figure 2.1 shows the data buffer image.