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Data Buffer Management
2 - 10 C141-E167
2.2.3 Look-Ahead operation, Look-Ahead volume
1) Excluding the conditions in 3) and 4) for the amount of data specified as the minimum pre-fetch
volume, data are read for look-ahead irrespective of track boundaries or cylinder boundaries.
Commands which are already in the queue or commands which are newly received while a look-
ahead operation is in progress, are executed after the look-ahead operation is completed.
However, if the new command is a READ or READ EXTENDED command, and it is a
command in which the first specified logical data block is a sequential access data block, the
command is executed without the look-ahead operation being halted. When a newly received
command is a command which is not the object of caching, that command is executed
immediately in parallel with a look-ahead operation.
Also, in the case of a command which disables all data which are objects of caching (see Section
2.2.1 (3)), the look-ahead operation is halted and that command is executed immediately.
2) When commands exist in the queue, the look-ahead operation is halted at the point when reading
of the specified amount of data at the minimum pre-fetch volume is completed, then execution
of the new command starts. When there are no commands that need to be newly executed, the
data look-ahead operation is continued, even after the volume of data specified as the minimum
amount of pre-fetch data have been read, until one of the following conditions, 1), 3) or 4),
occurs.
• When "1" is specified in the DISC bit, if look-ahead of data equal in volume to a cache
segment is completed.
• When "0" is specified in the DISC bit, if look-ahead of all the data is completed up to the
track boundary or the cylinder boundary.
The IDD does not support this function.
3) During a data look-ahead operation, when any error occurs, the data look-ahead operation is
terminated at that point (retry is not executed).
4) When a RESET condition occurs on the SCSI bus, or when any INIT issues a TARGET RESET
message, the look-ahead operation is terminated at that point and all the look-ahead data stored
in the data buffer are invalidated.
5) During a look-ahead operation, if "sector slip processing" is applied, or defective sectors exist in
the same cylinder to which "alternate processing" is applicable, the look-ahead operation is
continued without interruption.