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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
item 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.
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 BUS
DEVICE 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.
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C141-E039-01EN
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