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4.4 Various Processes
C156-E205-01EN 4-15
After automatic alternate assignment terminates successfully, the ODD does not
report any error.
4.4.5 Cache function
The ODD supports the 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 allows for data transfer at a near effective transfer speed
during continuous read operation without causing any delay owing to rotation
latency. LRU allows for data transfer with no mechanical operation if write and
read operations are executed frequently on the same sector.
The MO write cache writes data in batch mode during continuous write operation,
thus reducing positioning operations depending on the buffer size and recovering
the write throughput.
4.4.5.1 Data buffer
The ODD uses part of the buffer area as work memory for control firmware and
the remaining area as a buffer. The data buffer consists of plural segments and
contains write data of plural segments.
4.4.5.2 Read cache
The ODD enables the read ahead cache and LRU cache when the cache function is
enabled. When the read cache is enabled, the ODD contiguously pre-reads the
specified sector, then pre-reading the next sector when receiving a READ
command (called Read command in this document). While pre-reading the next
sector, the ODD transfers the data in the sector specified by the command.
When data in the sector specified by the host hits pre-read data, the ODD transfers
data directly from the buffer without any mechanical operation.
Pre-read stops under the following conditions:
!
Read error
!
When the data buffer is full (Pre-read may start again when the buffer
becomes sufficient.)
!
Media ejection instruction using the Eject switch
!
Power-off
!
When It is decided that the performance becomes better by stopping pre-
reading and give priority to other processings.
When the cache is enabled, the buffer contains data transferred from the host with
a write command. If a read command is issued to the sector address at this time,
the ODD directly transfers data to the high-level equipment from the data buffer