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CHAPTER 3 Data Buffer Management 3.1 Data Buffer
3.2 Look-Ahead Cache Feature
In this chapter, the configuration of the data buffer with which the HDD is
equipped, its operation and the operation of the cache feature are described.
3.1 Data Buffer
3.1.1 Data buffer configuration and basic operation
The HDD is equipped with a data buffer, which makes it possible to efficiently
execute data transfer operations between INIT (initiator) and a disk drive.
The HDD divides data buffer into 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 HDD automatically controls the best
suitable Cache Segment, INIT cannot be changed neither the Page size nor the
Segment size.
Each data buffer is a 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 interface (INIT). 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 HDD while being virtually
unaware of differences between the interface data transfer rate and the disk drive
data transfer rate.
Even if the data transfer capacity of the interface (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 interface (INIT) is higher than the disk drive's data transfer
rate, it is possible to minimize the occupancy time on the interface by
accumulating an appropriate amount of data in the data buffer.
Only one cache segment of the divided data buffer is used by one command and
the data in the other cache segments are held.
Figure 3.1 shows the data buffer image.