Date: Sun, 10 Oct 1999 13:26:56 -0700 (PDT) From: Julian Elischer <julian@whistle.com> To: Bruce Evans <bde@zeta.org.au> Cc: freebsd-arch@freebsd.org, mckusick@mckusick.com, committers@freebsd.org, current@freebsd.org Subject: Re: The eventual fate of BLOCK devices. Message-ID: <Pine.BSF.4.10.9910101323201.12493-100000@current1.whistle.com> In-Reply-To: <Pine.BSF.4.10.9910091626360.2053-100000@alphplex.bde.org>
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I Can't believe this email only produced TWO responses! I would have thought that this wouldhav brought out the chainsaws! Maybe no-one is listenning on 'arch' any more, or maybe 'arch' doesn't work? (the only responders got it via 'core') julian On Sat, 9 Oct 1999, Bruce Evans wrote: > > PHK has been moving steadily in this direction to remove as many > > dependencies within the kernel on block devices as possible. > > The question is, When did the decision to do so become official? > > Never. > > > I don't believe it has been a stated official decision yet and so in order > > to put some clarity into the air over this I'd like to launch a PURELY > > TECHNICAL discussion on the topic. > > > > Here are some starters. > > > > 1/ block device writes have to be synchrnous or the user doesn't get > > write errors. > > Block devices should be implmented properly or the user doesn't get write > errors. > > A proper implementation is quite close. Write errors should be reported > on last-close and on fsync(). They already are as far as I can see, modulo > the bugs that (in -current) VOP_FSYNC() = ffs_fsync() sometimes hangs > instead of returning a write error and vinvalbuf() sometimes panics instead > of returning a write error. The bugs are different and worse in RELENG_3. > The bugs are different and more benign in RELENG_2_2 (write errors are > ignored). Note that the bugs have very little to do with specfs. All > specfs can reasonably do is kill the endless retries at a suitable time, > probably after calling vinvalbuf() in last-close. > > > 1A/ if they are not synchronous, errors need to be coped with in some > > other manner. > > Normal error handling suffices, modulo bugs. > > > 2/ People with old UNIX experience expect to be able to do unalligned > > transfers on block devices. > > 3/ DEVFS can cope just fine with block and char devices > > (I include this because DEVFS has been used as an argument for > > removing them) > > Correct. > > > 4/ Most of the block buffering code in the kernel will remain due to > > the VM and VFS systems. > > Well, if the Nth rewrite of vm wants to drop support for buffers in vfs, > then use of buffers for block devices shouldn't stop it. > > > 5/ New users don't tend to understand the rather strange distinctions > > between BLK and CHR devices. Some people consider having both POLA and > > This is an argument for removing character (disk) devices, since most > new users will be from Linux where block (disk) devices were the only > ones available until recently. Block devices have always worked better > in Linux. E.g., media change is detected for floppies, and buffers > remain valid across last-close, until media change. The latter behaviour > can be not what is wanted (extra ioctls are needed to discard the buffers), > but it is often useful. > > > others consider having only one POLA. Linux had til just recently, > > only BLK disk devices. They just aded CHR disk devices but I don't > > know if they created a whole second calss of device to do so. (I doubt it) > > 6/ It should be possible to make an overlay device (similar to the way > > ccd works), that supplies buffered characteristics to a disk. This may > > be a different minor number or a differnt major number.. but be a CHR > > type device. > > This would involve needless duplicatication of half of the buffer cache > implementation (maybe the simple half) unless the buffer cache goes away. > > Bruce > > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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