Date: Wed, 16 Dec 1998 11:55:36 +1030 From: Greg Lehey <grog@lemis.com> To: mjacob@feral.com Cc: freebsd-current@FreeBSD.ORG Subject: Re: Tape Driver Changes Proposed: Tape Early Warning Behaviour Message-ID: <19981216115536.W15815@freebie.lemis.com> In-Reply-To: <Pine.LNX.4.04.9812141927130.2563-100000@feral-gw>; from Matthew Jacob on Mon, Dec 14, 1998 at 07:31:15PM -0800 References: <19981215135448.B15815@freebie.lemis.com> <Pine.LNX.4.04.9812141927130.2563-100000@feral-gw>
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On Monday, 14 December 1998 at 19:31:15 -0800, Matthew Jacob wrote: >> >> Well, you don't have to go overboard. Combine compression and >> density. We have BSD semantics, I suppose, so the only other thing is >> no rewind, which we already cater for. So for, say, an Exabyte >> 8505XL, you'd have: >> >> /dev/rst0l 8202 mode, no compression >> /dev/rst0n 8202 mode, compression >> /dev/rst0h 8205 mode, no compression >> /dev/rst0c 8205 mode, compression >> /dev/nrst0l 8202 mode, no compression, no rewind >> /dev/nrst0n 8202 mode, compression, no rewind >> /dev/nrst0h 8205 mode, no compression, no rewind >> /dev/nrst0c 8205 mode, compression, no rewind >> >> Sure, it's more than now, but it shouldn't confuse people too much. > > Cool, but there are four possible densities in the current data > structures. And speeds (has anyone actually ever found a drive that > *uses* these?). And multiple compression algorithms to select from. > > I'm kind of inclined to think that the compromise of > > Rew/Norew X Compress/Nocompress X Low/High density > > is sufficient as long as you can establish that the latter two > categories are a persistent (not through reboot, tho) cache of possible > values that you can set via the mt(1) command. I can't see any particular reason to restrict the minor number format if it's not necessary. So we have 4 densities, n (<=4?) speeds and compression. That makes 5 bits. Then we have non-rewind, a maximum of 16 units per drive and (on a PC) probably not more than 16 controllers. A total of 14 bits of minor number out of the 24 available: in other words, there should be no problem finding a minor number format which fits. The naming is a different matter. The `convenient' format doesn't have to cover every possibility; we could think up a `complete' format at a later date, something like /dev/tape/nrc0u5d3s1c meaning no rewind, raw, controller 0, unit 5, density setting 3, speed setting 1, compression. There's no reason to think these out at this stage, since they're probably not needed; but if we create a minor number format which can't represent one or the other of them, we're bound to find a case where it's needed. Greg -- See complete headers for address, home page and phone numbers finger grog@lemis.com for PGP public key To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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