Date: Wed, 11 Jul 2001 10:46:18 -0700 (PDT) From: Matthew Jacob <mjacob@feral.com> To: Joerg Wunsch <joerg_wunsch@uriah.heep.sax.de> Cc: freebsd-scsi@FreeBSD.ORG Subject: Re: Trouble with SCSI Streamer Message-ID: <Pine.BSF.4.21.0107111044250.5766-100000@beppo> In-Reply-To: <200107110654.f6B6sFp40894@uriah.heep.sax.de>
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On Wed, 11 Jul 2001, Joerg Wunsch wrote: > Matthew Jacob <mjacob@feral.com> wrote: > > > No, I don't agree. A lot of QIC tape drives don't get emulated (it > > has to be emulated because they cannot actually *do* variable > > blocksize) blocksizes right. > > We've been there before, Matt. /All/ current tape drives have to Yes, and I didn't agree with some of the conclusions. Yes, all tape drives 'emulate'. The QIC manufacturers, from my experience, don't do a good job of it. Let's cut this short. If you're willing to take over SA maintenance, please say so. I sure am not doing that great of a job. If you take it over, you can change the default to that which you thinkis best. > emulate variable length recording. The last drives that could do it > natively were the reel-to-reel half-inch drives, and Stephen probably > owns the single one of them that is working under FreeBSD. ;-) All > other drives, no exception, have some internal hardware blocking, > usually between 1 KB and 8 KB, for the media blocks. Yet, you don't > usually notice this since they all provide a successful illusion of > variable-length recording to the outside. > > QIC-320 and above are no exception to this. All of them provide a > variable length emulation. The only QIC drives that don't are (some) > QIC-150 drives (and even for those that do, you would not want to use > it there since the on-tape implementation is horrible and wasteful). > > So in general, you can safely default to variable length recording > except for QIC-150 media (and well, for QIC-24 (60 MB), but i doubt > anybody's using that anymore). This is what the pre-CAM tape driver > did, and nobody complained by that time. It just works, and is IMHO > what other operating systems are doing as well. > > There's one marginal difference in the variable-length emulation of > QIC vs. other media types, in that QIC can only handle at most one > logical block within a physical block. (*) So if you try to block your > tape with some odd size that is not a multiple of the 1 KB media block > size, you're going to waste space. Other media can stuff parts of a > logical block inside one media block. But this is a pathologic case, > useful and used block sizes for tapes are typically 10 or 32 KB > anyway. > > (*) No rule without an exception: if you use 512-byte fixed length > recording, they can stuff two logical records into one media block. > > -- > 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 > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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