Date: Mon, 15 Nov 1999 20:41:13 -0500 From: Greg Lehey <grog@mojave.sitaranetworks.com> To: mjacob@feral.com, "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net> Cc: current@FreeBSD.ORG Subject: Re: FreeBSD 4.0 SCSI Tape Driver Message-ID: <19991115204113.33800@mojave.sitaranetworks.com> In-Reply-To: <Pine.BSF.4.05.9911150931440.9192-100000@semuta.feral.com>; from Matthew Jacob on Mon, Nov 15, 1999 at 09:36:16AM -0800 References: <199911151715.JAA15320@gndrsh.dnsmgr.net> <Pine.BSF.4.05.9911150931440.9192-100000@semuta.feral.com>
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On Monday, 15 November 1999 at 9:36:16 -0800, Matthew Jacob wrote: >> >> There seems to be a great amount of confusion about the 2 EOF marks on >> tapes. It has nothing to do with physical EOT, even the 556BPI 1/2" >> tape drives on an IBM 1401 can detect physical EOT. The problem is >> with LOGICAL EOT, most tape drives do not have a logical EOT write >> command, even modern drives. So when you overwrite a tape how do you >> tell that you have gotten to the logical end of data, well, you write >> 2 EOF marks. >> >> The other thing that causes lots of folks confusion here is that some >> tape drives backspace over an EOF mark that is written, thus it gets >> real fun to put 2 EOF marks on the tape. You have to mt eof, mt fsf, >> mt eof. > > Yes, that *may* be a problem. Also, when you write two filemarks, as best > as I can tell for some hardware, this is never able to be read back as two > filemarks. > >> >> Since you do not point out how we are suppose to detect logical EOT >> on a tape I object to any elimination of dual EOF to indicate logical >> EOT. >>> >>> There already is an ioctl (and control via mt(1)) to change the default >>> eot model. There could very well also be a config option too. I'd like to >>> make the 1 Filemark at EOT the default though. I'll have to fix tcopy, >>> and I want to give some thought so that there are no compatibility >>> and interchange problems, but if those concerns are adequately covered I >>> think this is the right thing to do. >> >> 1 filemark can not be used for EOT, it is EOF, you can't tell if what you >> read next is another file or not that may have been left by a previosly >> longer usage on the tape. >> >>> >>> So- let me know, either via this list or privately. >>> Thanks in advance... >> >> Won't work, or would you care to explain how we are now suppose to detect >> logical EOT? > > The driver detects EOT during reads. Subsequent reads from the user > application return no data. A user application that detects a residual > twice in a row knows it is at EOT. Nearly all other Unix systems work fine > with this mechanism. Convince me. Every night, I do a partial backup, one file on tape for each file system, about 12 in all. Subsequently I read the tape and list contents until I hit EOT. OK, the first time I use a tape, there will be nothing behind it. But the next time, the total length of tape written may be shorter, so there will be data after logical EOT. How is the program going to know where to stop? Greg -- Finger grog@lemis.com for PGP public key See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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