Date: Wed, 11 Jul 2001 08:54:15 +0200 (MET DST) From: j@uriah.heep.sax.de (Joerg Wunsch) To: freebsd-scsi@freebsd.org Subject: Re: Trouble with SCSI Streamer Message-ID: <200107110654.f6B6sFp40894@uriah.heep.sax.de> References: <200107101252.f6ACqMF26033@dungeon.home> <20010710132824.C99370-100000@wonky.feral.com>
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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 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
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