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Drive Operations Chapter 3
Technical Reference Manual Page 25
Write Caching
Write Caching allows multiple write commands operating on sequential blocks to
be written to the medium without losing a motor revolution between commands.
Write caching is enabled by setting the WCE bit in MODE SELECT page 8 to
one. The WCE bit is only valid while the Read Look Ahead code is loaded. The
WCE bit is ignored when the Tagged Command Queuing code is in RAM because
write coalescing will be active.
The drive will send good status and COMMAND COMPLETE message following
the data out phase of a cached write command. The drive will cache writes when
the following conditions are met:
•
Two or more write commands (Op Code 0A
H
or 2A
H
) execute consecutively
without an intervening command.
•
The write commands address consecutive logical block ranges.
•
At least one logical block of data has been received in the buffer from the
second write command in time to allow the medium to be written before an
additional spindle revolution would be required.
•
Both writes are from the same initiator.
•
Neither write is a linked command.
If the drive encounters an error during a cached write operation, the drive will
respond by:
If AWRE (MODE SELECT page 01
H
) is 0: the drive will report a CHECK
CONDITION on the next command and the response from a REQUEST SENSE
will be a deferred error. (Asynchronous event notification is not supported by
this drive.) Refer to the Eleventh Generation SCSI Interface Manual for
additional details.
If AWRE (MODE SELECT page 01
H
) is set to 1: the drive will attempt to
dynamically reassign the block of data and complete the operation. If the
reassignment fails, the drive will continue to reassign the block until all the
space in the grown defect list is filled (147 sectors, maximum).