Fujitsu MBA3147RC Manuel D’Utilisation

C141-C013 

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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. 

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.