Re: tlug: Floppy mounts

[Frank Bennett <bennett@example.com>]

Frank Bennett <bennett@example.com> writes:

> For the mo, tho, looks like mtools is what I'm after.

Fear not, I've not started talking to myself again.  I've quoted
the above so I can point out that I'm wavering on the approach to
take to this problem.

When Rex pointed out mtools and I ran a couple of the utilities,
I thought that that clinched it.  It looked really easy and
straightforward to me, but then I thought again.  The users I'm
faced with are Windows children, not DOS children.  They will not
know what to do with a command line; they will expect the
filemanager to offer all the options.

So mtools handles hardware the way I want it handled, but it
doesn't offer the right user interface, and there's no way to
strap that interface onto it that I can think of; the DOS floppy
has got to look like an extension of the Unix filesystem, so that
any Unix filemanager can get ahold of it.  And the only way to
accomplish that is by mounting it.

So I played around with mount -o sync for a couple of minutes,
and discovered that mount and the floppy driver are more robust
than I expected them to be.  I ran the following sequence:

  mount -t msdos -o sync /dev/fd0 /mnt/floppy
  ls -l /mnt/floppy
  [remove disk A, insert disk B]
  ls -l /mnt/floppy
  mount -t msdos -o sync /dev/fd0 /mnt/floppy
  ls -l /mnt/floppy

The results are interesting, and likely useful.  The second ls -l
showed a cached listing of the removed disk -- not good, because
things are then out of step.  The second mount command returns a
couple of nasty looking error messages (fstab shows filesystem
already mounted, that sort of thing).  But despite this, the
third ls -l actually shows the new filesystem.

Apparently because the disk is known by the OS to be synced, it
will whingingly allow you to overlay a new mount onto its mount
point without unmounting the disk that's been taken out.

Next, I tried this:

  mount -t msdos -o sync /dev/fd0 /mnt/floppy
  ls -l /mnt/floppy
  [remove disk A, insert disk B]
  echo WOW > /mnt/floppy/test
  ls -l /mnt/floppy
  [replace disk A]
  ls -l /mnt/floppy
  [remove disk A, insert disk B]
  mount -t msdos -o sync /dev/fd0 /mnt/floppy
  ls -l /mnt/floppy

(Sorry I can't show the actual progress: cut and paste are broken
in my X display right now.)

Here, the second ls -l shows the new file /mnt/floppy/test in the
filesystem.  No lights come on, so I thought "wow, sync doesn't
work".  So I replaced the original disk, and did the third ls -l.
The disk ground a little bit, and then I got a listing WITHOUT
THE NEW FILE.  This is the right behavior; the write failed, so
no file exists.

The rest of the test ran as in the previous trial; the overlaid
mount took effect after the OS got done whinging about the ugly
way the hardware was being abused.  Just to be sure, I tried this
same sequence again, but without the reinsertion of Disk A.  All
worked as it ought; "test" just vanished into the ether.

So it looks like automounting will probably be reliable enough
for our needs, once I get it working.  If the timeout for mounts
is just a bit less than the time it takes to switch disks, users
will have to work pretty hard to get everything in place in time
for a file write to disappear on them.

Many thanks to Howard, Rex and Scott for helping me thrash
through this.
-- 
-x80
Frank G Bennett, Jr         @@
Faculty of Law, Nagoya Univ () email: bennett@example.com
Tel: +81[(0)52]789-2239     () WWW:   http://rumple.soas.ac.uk/~bennett/
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