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
>
>
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