From owner-freebsd-scsi@FreeBSD.ORG Mon Oct 10 22:42:05 2005 Return-Path: X-Original-To: freebsd-scsi@freebsd.org Delivered-To: freebsd-scsi@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 1F99616A41F for ; Mon, 10 Oct 2005 22:42:05 +0000 (GMT) (envelope-from roy+dated+1131576117.13826b@gnomon.org.uk) Received: from darla.gnomon.org.uk (darla.gnomon.org.uk [212.69.37.162]) by mx1.FreeBSD.org (Postfix) with ESMTP id 5E3EE43D46 for ; Mon, 10 Oct 2005 22:42:04 +0000 (GMT) (envelope-from roy+dated+1131576117.13826b@gnomon.org.uk) Received: from giles.gnomon.org.uk (cpc2-cmbg2-5-0-cust198.cmbg.cable.ntl.com [80.4.124.198]) by darla.gnomon.org.uk (8.13.4/8.13.4) with ESMTP id j9AMfvTD042715 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Mon, 10 Oct 2005 23:42:03 +0100 (BST) (envelope-from roy+dated+1131576117.13826b@gnomon.org.uk) Received: from giles.gnomon.org.uk (localhost.gnomon.org.uk [127.0.0.1]) by giles.gnomon.org.uk (8.13.2/8.13.0) with ESMTP id j9AMfv2C015625 for ; Mon, 10 Oct 2005 23:41:57 +0100 (BST) (envelope-from roy+dated+1131576117.13826b@giles.gnomon.org.uk) Received: (from roy@localhost) by giles.gnomon.org.uk (8.13.2/8.13.0/Submit) id j9AMfv6b015624 for freebsd-scsi@freebsd.org; Mon, 10 Oct 2005 23:41:57 +0100 (BST) (envelope-from roy+dated+1131576117.13826b@giles.gnomon.org.uk) Received: by giles.gnomon.org.uk (tmda-sendmail, from uid 559); Mon, 10 Oct 2005 23:41:57 +0100 (BST) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <17226.60980.856228.713566@giles.gnomon.org.uk> Date: Mon, 10 Oct 2005 23:41:56 +0100 To: freebsd-scsi@freebsd.org In-Reply-To: <17226.58019.311383.858203@giles.gnomon.org.uk> References: <17226.58019.311383.858203@giles.gnomon.org.uk> X-Mailer: VM 7.19 under Emacs 21.3.1 From: Roy Badami X-Delivery-Agent: TMDA/1.0.3 (Seattle Slew) X-Primary-Address: roy@gnomon.org.uk Received-SPF: pass (darla.gnomon.org.uk: 80.4.124.198 is authenticated by a trusted mechanism) Subject: QIC tapes and simulated double filemarks X-BeenThere: freebsd-scsi@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: SCSI subsystem List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 10 Oct 2005 22:42:05 -0000 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