more Exabyte lore

[roger willcocks]

the Exabyte drive manual says, under the write filemark command:

>    Short Filemark: this Filemark is a non-eraseable filemark. It reduces
>    the amount of tape used.
>
>    Short Filemarks may not be overwritten by data or another Filemark,
>    short or long. Positioning is the same for both short and long
>    Filemarks. Data can be appended to the end of tape side of a short
>    Filemark if at blank tape.

What it doesn't say is how much tape is used by a 'long' filemark.
According to Phase-IV (UK distributors - helpful people) the answer is
90mm or so, i.e.  2.1 Megabytes worth. This may significantly damage your
capacity. A long filemark takes about 10 seconds to read.

The whole scoop: long filemarks use 271 helical scans worth (1 scan == 8
Kb) short filemarks use 61 scans worth, the tape is read at 246 Kb/s,
seeking is 10 times faster than reading.

All this is due to the data being written helically but the erase head
clearing the entire width of the tape as it passes ...

                     //////  #                  / : data being written
                    //////   #                  # : position of erase head
                   //////    #

nThere is no way to erase from a particular helical scan; some part of it
will always be left (and the tape corrupted).

The trick is to arrange that there is a big hole before a filemark, in
which the erase head can be parked.

                     //////  #    X///           X : filemark
                    //////   #   X///
                   //////    #  X///

This hole is 90mm long. 

Short filemarks don't have such a hole, so there is no way to position the
tape so that writing will work properly.

There is no hole after either flavour of filemark. So the only time you
can write data after a filemark is when the filemark was the last thing
that was written to the tape.

But of course, you can seek to the BOT side of a long mark, rewrite the
mark and then your data.

Anyhow watch out if you'll want to seek to not-the-last-filemark and then
write, because it HAS to be a long one. And it uses up a lot of tape.

Disclaimer: These are my own opinions and blah blah blah.

Roger Willcocks, Principal Wizard
Crosfield Electronics Ltd
Hemel Hempstead, England		rkww at cel.co.uk
+44 442 230000				mcvax!cel!rkww at uunet.uu.net