Fujitsu MAP3147 Manual Do Utilizador
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.
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.
2.2.1 (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.
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.
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.
continued without interruption.