Fujitsu MAN3735 Manuale Utente
C141-E124-01EN
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2.2
Look-Ahead Cache Feature
In order to use the data buffer more effectively and improve the disk drive's effective access speed,
the IDD is equipped with a simple cache feature called a "Look-Ahead Cache Feature." The Look-
Ahead cache feature is an effective, simple cache function for an INIT which reads data block
groups on the disk media sequentially using multiple commands.
the IDD is equipped with a simple cache feature called a "Look-Ahead Cache Feature." The Look-
Ahead cache feature is an effective, simple cache function for an INIT which reads data block
groups on the disk media sequentially using multiple commands.
2.2.1
Caching operation
When the IDD is executing a READ command or a READ EXTENDED command, it reads the
requested data from the disk media and transfers it to the INIT while at the same time reading (looking
ahead at) data blocks which are subsequent to the last logical data block specified in that command to the
data buffer. Then, when a READ command or READ EXTENDED command issued later specifies any
of those Look-Ahead data blocks, the IDD transfers those data directly to the INIT from the data buffer
without accessing the disk media. In this way, when continuous logical data block groups are read
sequentially by multiple commands, mechanical access operations can be eliminated and effective access
time greatly reduced.
requested data from the disk media and transfers it to the INIT while at the same time reading (looking
ahead at) data blocks which are subsequent to the last logical data block specified in that command to the
data buffer. Then, when a READ command or READ EXTENDED command issued later specifies any
of those Look-Ahead data blocks, the IDD transfers those data directly to the INIT from the data buffer
without accessing the disk media. In this way, when continuous logical data block groups are read
sequentially by multiple commands, mechanical access operations can be eliminated and effective access
time greatly reduced.
An INIT can prohibit the operation of this Look-Ahead cache feature by setting the caching
parameter described in section 2.2.2.
parameter described in section 2.2.2.
(1)
Caching object commands
Commands which execute caching operations are the commands shown below:
x READ
x READ EXTENDED
x READ EXTENDED
If all the data block groups which are objects of the processing specified in these commands, or a
portion of the data blocks, including the top logical data block specified in these commands are
relevant data (See item (2)) for caching in the data buffer (if they hit), when the IDD receives a
command and performs disconnect processing, it immediately reconnects and sends the data on the
data buffer which can be transferred to the INIT. The caching operation can be disabled by the
FUA bit of the READ EXTENDED command. If "1" is specified in the FUA bit, the IDD reads
data from the disk media.
portion of the data blocks, including the top logical data block specified in these commands are
relevant data (See item (2)) for caching in the data buffer (if they hit), when the IDD receives a
command and performs disconnect processing, it immediately reconnects and sends the data on the
data buffer which can be transferred to the INIT. The caching operation can be disabled by the
FUA bit of the READ EXTENDED command. If "1" is specified in the FUA bit, the IDD reads
data from the disk media.
On the other hand, if the top logical data block specified in a command is not relevant data for
caching in the data buffer (if they miss), reading of data is performed from the disk media using the
old cache segment.
caching in the data buffer (if they miss), reading of data is performed from the disk media using the
old cache segment.
(2)
Data which are objects of caching
Data which exist in the data buffer and which are objects of caching in item (1) (data which are not
accessed from the disk media but are transferred to the INIT from the data buffer) are as described
below.
accessed from the disk media but are transferred to the INIT from the data buffer) are as described
below.
a) Data read by a READ or a READ EXTENDED command and which have been read to the
data buffer by Look-Ahead are data which are objects of caching. Depending on the timing for
halting Look-Ahead, it is possible that data read by the READ command will be overwritten by
Look-Ahead.
halting Look-Ahead, it is possible that data read by the READ command will be overwritten by
Look-Ahead.