Buffered Mode - Tandberg Data SLR7-140 PROGRAMMER Manual

Tandberg Data
The size of a logical data block may be different than
the size of a physical data block. In the tape buffer
the size of a physical block is fixed (as given by the
current tape format). In the host buffer the size of a
logical block may vary since the Drive supports a wide
range of fixed- and variable logical block sizes. The
required blocking and de-blocking into logical blocks is
handled by the data-formatting unit.
When the data
logical block usually occupies at least one complete
physical block. There is one exception. When writing 512
byte blocks with the FIXED bit set to one, a special op-
tion is used on SLR5 and DC9250 media allowing two
logical blocks to share one physical block [9] - [10].
Large logical blocks may span several physical blocks.
When data compression is used, the number of physical
blocks needed for each logical block depends on the
compression ratio achieved.
2.7.3.

Buffered Mode

The Drive is capable of operating in both a buffered
mode and an unbuffered
applicable during
buffered mode the Drive returns GOOD status for write
operations when
transferred from the Initiator into the data buffer.
When operating in unbuffered mode, GOOD status is not
returned until
successfully recorded on the medium.
When issuing a buffered WRITE FILEMARKS command with the
immediate bit set to one, GOOD status is returned as
soon as the tapemark(s) has been moved into the data
buffer. A WRITE FILEMARKS command with the immediate bit
set to zero causes any buffered data blocks or tapemarks
to be written to the medium. Upon successful completion
of this process,
operation, no data blocks or tapemarks remain in the
data buffer. (Host
operation without writing actual tapemarks by specifying
zero in the number
FILEMARK command).
Should an unrecoverable
buffered mode, the Drive generates an error condition.
The error is reported on the next applicable operation
as a deferred error.
Note that when operating in buffered mode the Drive will
pack physical blocks from one WRITE or WRITE FILEMARKS
command together with physical blocks from the previous
WRITE or WRITE FILEMARKS command into the same frame
(when a frame based tape format is used). This is not
true when operating in non-buffered mode. If a WRITE or
WRITE FILEMARKS command does not transfer enough data
(or tapemarks) to fill a complete frame, the rest of the
frame is filled up with filler blocks. This may of
course waste a lot of space on a tape. When reading a
tape containing frames with filler blocks the Drive is
able to remove the filler blocks without generating any
discontinuities
Note, however, that even if filler blocks are inserted
2-23 Tandberg SLR Product Line SCSI Functional Specifications
compression option
mode.
read commands.
all data have
all requested data
which is
may perform
of tapemark
write
in the data stream
About Tape Streamers
is not used,
Buffered mode is
When operating
been successfully
or tapemarks
called a synchronize
this synchronize
field in the
error occur while
on the SCSI-bus.
one
not
in
are
WRITE
in