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4.2.4 Cache function
The ODD supports read cache and MO write cache.
The read cache consists of the read-ahead cache and the LRU cache that reads
write data remaining in the data buffer.
The read-ahead cache enables data transfer at almost the same speed as the
effective transfer speed during continuous read operation without causing delays
resulting from rotation latency. If write and read operations are executed
frequently on a same sector, the LRU cache enables the transfer of data without
mechanical operation.
The MO write cache writes data in batch mode during continuous write operation,
thus reducing the number of positioning operations, which is determined by the
buffer size, and improving write throughput.
4.2.4.1 Data buffer
The ODD uses part of the buffer area as work memory for control firmware and
the remaining area for a data buffer. The data buffer consists of multiple segments
and contains multiple segments of write data.
4.2.4.2 Read cache
If the read cache function is enabled, the ODD enables the read-ahead cache and
LRU cache. When a READ command is received while the read cache is enabled,
the ODD reads the specified sector and continues pre-reading the following
sectors. The ODD thus transfers data in the sectors specified by the command
while pre-reading the sectors that follow.
When data in the sector specified by the host is pre-read data, the ODD transfers
the data directly from the buffer without any mechanical operation.
The ODD stops pre-reading under any of the following conditions:
•
Read error
•
Data buffer full (The ODD may restart pre-reading when the buffer is no
longer full.)
•
Media ejection instruction given with the eject switch
•
Power-off
•
The system judges that performance will be improved by stopping the pre-
reading and giving priority to other processing.
If the read cache is enabled, the buffer contains data transferred from the host with
a write command. If a read command is issued for an applicable sector address,
the ODD directly transfers data to the host from the data buffer rather than reading
it from the media. If the cache is hit with a read command, the ODD places and
keeps the data in the hit segment in the highest priority.
The following types of data are not subject to the read cache function:
C156-E224-01EN
4.2 Various Processes
4-5
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