Date: Mon, 10 Oct 2005 23:41:56 +0100 From: Roy Badami <roy@gnomon.org.uk> To: freebsd-scsi@freebsd.org Subject: QIC tapes and simulated double filemarks Message-ID: <17226.60980.856228.713566@giles.gnomon.org.uk> In-Reply-To: <17226.58019.311383.858203@giles.gnomon.org.uk> References: <17226.58019.311383.858203@giles.gnomon.org.uk>
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Actually, on re-reading that I now have doubts about some of what I
wrote...
Roy> However, this means that the drive can't support overwriting
Roy> from an arbitrary position on the tape; unless you were
Roy> positioned on track 1, the erase head wouldn't be energised
Roy> and hence the data would be written on tape that hadn't been
Roy> erased, and would be garbled.
Actually, the design of the drive, with the moveable read/write head
and a fixed erase head means there's a significant distance between
the two. I suspect that means that overwriting (other than from BOM
or EOD) wouldn't be possible even on track 1, because the tape between
the read/write head and the erase head still wouldn't be erased.
Roy> However, the next workstation tape technology that came along
Roy> was the Exabyte 8mm tape, essentially Video 8, which lacked
Roy> the overwrite restrictions of QIC (not being serpentine)
I'm now having doubts about whether early Exabyte drives supported
overwriting from an arbitary position on the tape. First generation
drives (Exabyte 8200) were very closely related to Video 8 technology,
and indeed you could (and people did) get away with using standard
Video 8 tapes rather than the more expensive cartridges that Exabyte
sold. I suspect they may well have used a fixed erase head (much like
VHS) and had similar restrictions; I honestly don't know one way or
the other, and I don't have access to a working 8200 drive to do the
test. Of course, given the Exabyte drives abstracted the logical tape
format away from the physical, it's possible that phantom filemarks
were handled by the drive, if indeed they were necessary.
-roy
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