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6.4 Read-ahead Cache
C141-E262 6-19
6.4.3.4 Partial hit
In this situation, a part of read requested data including the top sector is stored in
the data buffer. A transfer of the read requested data starts from the address where
the data that is hit is stored until the top sector of the read requested data.
Remaining part of insufficient data is read then.
An example is a case where a partial hit occurs in cache data, as shown below.
Cache valid data
START LBA
LAST LBA
1) HAP is set at the address where partial hit data is stored, and Transfer is
started.
Cache valid data Partial hit data
HAP (host address
p
ointer)
2) DAP and HAP are set at the head of Buffer newly allocated, and insufficient
data is read.
Read segment
HAP (host address
p
ointer)
DAP (disk address pointer)
3) When reading the read requested data ends and the transmission of the read
requested data to the host system ends, the read-ahead operation continues
until a certain amount of data is stored.
The method of storing the read-ahead data at Partial hit is the same as the
Miss hit.
Cache valid data
START LBALAST LBA