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2.2 Look-Ahead Cache Feature
C141-E167 2 - 7
(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 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.
(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.
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.
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.
c) Data transferred from the INIT and written to the disk media by the WRTTE and WRITE
EXTENDED command are objects of caching.
d) Data transferred from the INIT and written to the disk media by the WRITE AND VERIFY
command is not the objects of caching.
(3) Disabling caching data
Various data which are objects of caching in the data buffer are disabled in the following cases.
a) If any of the following commands is issued for the same data block as data which are the object
of caching, that data block ceases to be an object of caching.
•
WRITE
•
WRITE AND VERIFY
•
WRITE EXTENDED