Exabyte tapes - variable vs fixed blocks

[andrew yeomans]

(I never saw a definitive answer on this subject, so I'll pass on what I
discovered).

The Exabyte 8mm drive supports both 'fixed block mode' and 'variable block
mode'.  These two modes are selected by SCSI commands (however, your
device driver may hide this from you). The read and write commands also
specify the mode, and this MUST agree with the selected mode.

These modes ONLY affect the SCSI interface mode - the data recorded on
tape is the same in both modes.  The data stored on tape (effectively)
contains the recorded record length, which is checked when reading back.
If this does not agree with the record length requested in the read, an
ILI (illegal length indicator) error is reported, unless this has been
suppressed by the SIL (suppress illegal length) command.  The number of
bytes returned is the minimum of the requested length or the recorded
length.

Thus you may record 2000 byte records in variable mode, set the mode to
fixed with block size of 2000, and read back the data without errors.

It is up to the device driver to determine whether ILI is acceptable or
is an error.  A reasonable device driver would return an error if
recorded length > requested, but not vice versa.

I said the data _effectively_ contains record length.  Strictly speaking,
the logical records are divided up into 1 Kbyte physical records.  Each
physical record has its length (1-1024) stored, together with two flag
bits S and E meaning 'this physical record is the start/end of a logical
block'.  The last block is padded to 1024 bytes if necessary.  Short
logical records are not packed into a single pysical record.  Luckily, the
controller hides all this!

If you are really interested in the tape format, Ansi committee X3B5 is
proposing to standardise the Exabyte format.  See document X3B5/88-186A
"Helical-scan digital computer tape cartridge 8mm (0.315 in) for
information interchange" for full details (you'll probably regret it!).

Advantages of variable mode: Tape behaves like a 9-track tape.

Advantages of fixed mode: Reduced SCSI command overhead - can send many
records with one command (providing all are the same size).

 Andrew Yeomans             PSTN: 0442 230000 ext 3371
 Crosfield Electronics Ltd  INTL: +44 442 230000      
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