Fujitsu MAN3735 Manuel D’Utilisation
C141-E124-01EN
2 - 10
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.
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.
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.
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.
x When "1" is specified in the DISC bit, if look-ahead of data equal in volume to a cache
segment is completed.
x 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.
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.
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.
continued without interruption.