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Supermount README
=================
For supermount v0.4
Supermount is a pseudo-filesystem which manages filesystems on
removable media like floppy disks and CD-ROMs. It aims to make
management of removable media as easy as it is under DOS.
With supermount, you can change the disk in the drive whenever you
want (with the obvious exception that you shouldn't do it when the
filesystem is actively in use). You don't need to "cd" out of the
directory first, and you don't need to tell the kernel what you're
doing --- supermount will detect the media change automatically.
Supermount will automatically detect whether the media you are
mounting is read-write or readonly, and if you mount a write-protected
disk, then the subfs will be mounted as a readonly filesystem.
Supermount detects when you have finished activity on the subfs, and
will flush all buffers to the disk before completing the operation.
So, if you copy a file onto a supermounted floppy disk, the data will
all be written to disk before the "cp" command finishes. When the
command does complete, it will be safe to remove the disk.
It is worth while defining what I mean by "activity" here. The subfs
is active if there are any processes running which have a handle on a
non-directory inode on the subfs, or which have a file open on the
subfs (even if only for reading). There is one important case which
does NOT count as activity: if you "cd" to a directory or a
subdirectory under the supermount mount point, then that reference
does not make the subfs active, and you can safely remove the disk.
Yes, that's right. You can "cd /floppy; ls" and get a listing of a
dos floppy. Remove the disk, insert a new one, and "ls" will now list
the new contents. Remove the disk altogether, and "ls" will give you
an I/O error. Put in a new disk, and "ls" starts working again. It
is NOT an error to remove a supermounted disk which is acting as the
current working directory for one or more processes!
The Superfilesystem and Subfilesystem concepts
----------------------------------------------
Normally, when you mount a filesystem, you create a direct connection
between a mount point in the directory tree and a filesystem on a
block device. Supermount adds an extra layer in between; with
supermount, you explicitly mount the pseudo-filesystem (the
"superfilesystem") on the mount point, and supermount then
automatically mounts the real filesystem (the "subfilesystem") on the
block device when needed.
Running supermount
------------------
To run supermount, compile and install a kernel with the supermount
patches and select "Y" to the question
Dynamic mounting of removable media?
when you run "make config". You set up a supermount filesystem with
the normal mount command, using the syntax:
mount -o <superfs-options>,--,<subfs-options> / <mount-point>
Notice that I specified "/" instead of giving a block device to
supermount. This is because the supermount filesystem is NOT
connected to a block device; rather, supermount is responsible for
connecting a separate filesystem to the block device. You specify
this by providing the <superfs-options> field, where the following
options are currently recognised:
* subfs=<filesystem-type> [default is "msdos"]
Specify the subfilesystem type. "msdos" and "iso9660" have
been tested, but any block-device filesystem should work with one
important restriction: the filesystem must NOT try to write to the
device when you unmount it. This is because supermount doesn't know
in advance when it will have to unmount the subfs, so it doesn't try
to do so until it detects that the media has been changed. By this
time it is too late to write to the device!
If you mount supermount as a readonly filesystem ("mount -r"
or "mount -o ro"), then you won't have this problem. Otherwise, you
will not be able to use the ext2fs or minix filesystems with
supermount. This will hopefully be addressed in a future release of
supermount.
* dev=<block-device> [default is "/dev/fd0"]
Specify the block device on which the subfs is to be mounted.
* debug
Enable debugging code in the supermount filesystem, if
the debug option was enabled at compile time. By default, debugging
code is compiled into the kernel but is disabled until a debug mount
option is seen.
* '--'
All options after the option string '--' will be passed
directly to the subfilesystem when it gets mounted.
Here is an example of supermount options, taken directly out of my
current /etc/fstab:
/ /floppy supermount --,gid=51,conv=binary 0 0
/ /cd supermount ro,fs=iso9660,dev=/dev/hdd,--,conv=binary 0 0
This tells supermount to manage a msdos filesystem on /dev/fd0 mounted
at /floppy (msdos and /dev/fd0 are defaults for supermount), with the
msdos filesystem getting the options "gid=51,conv=binary". My cdrom
on /dev/hdd is similarly mounted on /cd.
Enjoy supermount. I hope you find it useful --- I certainly find it
extremely convenient. Send any comments, bug-fixes or bug-reports,
suggestions and success stories to sct@dcs.ed.ac.uk. Flames to
/dev/null, please!
Cheers,
Stephen.
--
Stephen Tweedie <sct@dcs.ed.ac.uk>
Department of Computer Science, Edinburgh University, Scotland.
================================================================
Changes for v0.4:
Performance tuning only. Read-only operations like "find" should now
be MUCH faster.
Changes for v0.3:
Fixed supermount_create bug; now returns a properly attached
superinode.
Changes for v0.2:
Improved device invalidation code, so CD-ROMs work now.
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