Fujitsu MAT3300FC 用户手册
Data Buffer Management
3-8
C141-C008
3.2.1 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.
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.
An INIT can prohibit the operation of this Look-Ahead cache feature by setting
the caching parameter described in section 3.2.2.
the caching parameter described in section 3.2.2.
(1) Caching object commands
Commands which execute caching operations are the commands shown below:
•
READ
•
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, it immediately opens
the loop 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.
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, it immediately opens
the loop 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.
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.
(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.
(data which are not 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.
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.
b) Data which have been hit by the READ or READ EXTENDED command
and transferred to the INIT once are also objects of caching as long as they
are not invalidated.
are not invalidated.