Date: Wed, 6 Nov 2002 10:58:11 +0100 From: Joerg Wunsch <j@uriah.heep.sax.de> To: Ruslan Ermilov <ru@freebsd.org> Cc: freebsd-scsi@freebsd.org Subject: Re: ``mt erase 0'' on a non-rewinded tape Message-ID: <20021106105811.A31642@uriah.heep.sax.de> In-Reply-To: <20021106092223.GB93420@sunbay.com>; from ru@freebsd.org on Wed, Nov 06, 2002 at 11:22:23AM %2B0200 References: <20021105122423.GA79188@sunbay.com> <200211051918.gA5JIqaY014094@uriah.heep.sax.de> <20021106092223.GB93420@sunbay.com>
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As Ruslan Ermilov wrote: > level 0 backups at Friday's evening, level 1 backups on Monday, ..., > level 4 backups on Thursday. If I don't erase the tape when I do a > level 0 backup, won't there be a chance that when I later do an > incremental backup and do an "mt eom" it will find some stale file > markers left by old incremental backups? Nope. When you start writing from the beginning of the medium, it'll always set a new EOM. You can't access the old contents behind EOM (at least not using the normal SCSI interface -- the only known method is to poweroff the drive after writing behind the previous EOM, or to hand off your medium to the NSA ;-). That's not half-inch reel-to-reel tapes anymore :), they are the only drives that needed two file marks in order to mark the logical end of tape, since you could still position behind it, and continue reading. Well, we still write two filemarks anyway, but other than compatibility with prehistoric software, there's no practical reason for this since the drives/media of all modern drives have their own notion of EOM anyway. > I wasn't clear enough. I meant it worked if I always used the rewind > device. In this case, "mt erase" was always run at the BOM. If the tape is at BOM, the command is supposed to work regardless of whether you use /dev/sa0 or /dev/nsa0. If it doesn't, that should be debugged. > Yes, the script runs "mt eom" for that very reason. But why would I > rewind the tape if the next day I want to use it from this same > position? IOW, if the SCSI bus isn't reset in-between, I get: > > # /usr/bin/time mt eom > 0,01 real 0,00 user 0,00 sys OK, saving a minute of positioning time might be an argument. :) -- cheers, J"org .-.-. --... ...-- -.. . DL8DTL http://www.sax.de/~joerg/ NIC: JW11-RIPE Never trust an operating system you don't have sources for. ;-) To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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