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Tandberg SLR Reference Manual
7-1
7.
Basic Operational Functions
7.1. Overwriting Previous Data
The QIC-standards do not allow writing to the tape except when positioned as described in
the two previous sections. The Drive does, however, support
logical overwrite
at two defined
places:
1)
after the first data block
2)
before the last Filemark in front of EOD
This makes it possible to emulate the kind of overwrite that is necessary to support the old
TAR-format.
When overwriting after the 1st data block, the tape will be entirely rewritten, and the first block
will be written to the tape before any new data is added.
When “overwriting” the last Filemark, the Filemark is not physically overwritten, but a
Filemark
Cancel Block
is written instead as the first appended block. This
Cancel Block
will, when
reading logically, cancel the previous Filemark. (For more information, see the SCSI-interface
documents).
7.2. Terminate Write From a Position on the Tape
A single- or sequences of Write Data or Write Filemark commands can be terminated at any
position on the tape. A termination is true if any other command that repositions the tape -
except new Write commands - is performed.
At this point the Drive will fill incomplete frames with
Filler Blocks
to assure that only complete
frames are written to the tape. After the last frame a set of EOD blocks are written to the tape
before the tape is moved away from the EOD position.
QIC-2GB and
QIC-4GB
The Filler Blocks will, when the format is QIC-2GB and QIC-4GB, contain a
“Block Map”
,
which is a kind of directory handled by the Drive, totally transparent to the user. It will allow
fast access (seeking) to any logical block, Filemark or Setmark. If there is not sufficient Filler
Blocks in the last frame to hold the Block Map, a complete frame with only Filler Blocks will
be added.
If the termination is on the first track, and the format is QIC-2GB or QIC-4GB, the data will
automatically be terminated with at least 45 inches of erased tape after the last written EOD
block before executing the next command. This is done transparently to the Host and
assures that the End Of Data can be detected unambiguously at Read time, even if the tape
contained data before the Write operation. (See Section 6.8. Data Append.)