Write Cache - Fujitsu MHA2021AT Product Manual

Operations

6.6 Write Cache

The write cache function of the drive makes a high speed processing in the case
that data to be written by a write command is physically sequent the data of
previous command and random write operation is performed.
When the drive receives a write command, the drive starts transferring data of
sectors requested by the host system and writing on the disk medium. After
transferring data of sectors requested by the host system, the drive generates the
interrupt of command complete. Also, the drive sets the normal end status in the
Status register.
The drive continues writing data on the disk medium. When all data requested by
the host are written on the disk medium, actual write operation is completed.
The drive receives the next command continuously. If the received command is a
"sequential write" (data to be written by a command is physically sequent to data
of previous command), the drive starts data transfer and receives data of sectors
requested by the host system. At this time, if the write operation of the previous
command is still been executed, the drive continuously executes the write
operation of the next command from the sector next to the last sector of the
previous write operation. Thus, the latency time for detecting a target sector of
the next command is eliminated. This shortens the access time.
The drive generates an interrupt of command complete after completion of data
transfer requested by the host system as same as at previous command.
When the write operation of the previous command had been completed, the
latency time occurs to search the target sector.
If the received command is not a "sequential write", the drive receives data of sectors
requested by the host system as same as "sequential write". The drive generates the
interrupt of command complete after completion of data transfer requested by the host
system. Received data is processed after completion of the write operation to the disk
medium of the previous command.
Even if a hard reset or soft reset is received or the write cache function is disabled
by the SET FEATURES command during unwritten data is kept, the instruction is
not enabled until remaining unwritten data is written onto the disk medium.
The drive uses a cache data of the last write command as a read cache data. When
a read command is issued to the same address after the write command (cache
hit), the read operation to the disk medium is not performed.
If an error occurs during the write operation, the device retries the processing. If the
error cannot be recovered by retry, automatic alternate assignment is performed. For
details about automate alternate assignment, see item (3) of Section 6.42.
The write cache function is operated with the following command.
6-22 C141-E042-01EN