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Re: nested file system types
On Mon, 16 Mar 2009 08:08:54 -0400
Greg Troxel <gdt%ir.bbn.com@localhost> wrote:
> "Toru Nishimura" <locore64%alkyltechnology.com@localhost> writes:
> > Hi,
> >> I'm working on mount_vnd(8). It's pretty
> >> straight-forward; the catch is that I need to be able to specify
> >> the file system type that's layered on top of vnd.
> > I'm skeptic about inventing mount_vnd(8), however, want to wait
> > for the output. If going that way, please consider how /etc/fstab
> > should be sanely extended for multiple stacked FS. One of
> > fundamental issues we have is a vague distinction (or confusion)
> > among "filesystem" and "how-to-stack-each-other."
> I don't understand from the name what mount_vnd is supposed to do.
> There is now a three step process:
> find a free vnd somehow
> vnconfig (make vnd associated with a file)
> mount the vnd
> so what I think would be cool is extending
> mount_lfs /dev/rwd0e /mnt
> mount_lfs vnd:/home/gdt/lfs.image /mnt
> where mount_msdos grabs a vnd number and does vnconfig, and umount
I assume you meant lfs there.
> will do vnconfig -u and release.
This is more or less it. I want the moral equivalent of ad's
mount -t cd9660 image.iso /mnt
but as he notes, that involves "a lot of tedious work". Worse yet,
it's kernel work, and if we have the ability to do it in userland I
don't see why we should add more complexity to the kernel.
The rationale for the functionality is simple: this is a very common use
(arguably, the most common use) of vnds; why not make it easy?
One option is to change mount to detect that the first positional
parameter is a file rather than a device. I'm very reluctant to do
that, because if you make a typing mistake or get the order wrong
you're in big trouble. Instead, I was originally thinking of something
mount_vnd -T lfs [-r] [-z] file mountpoint
Finding a free vnd is unpleasant but not difficult. If nothing else,
the code can iterate, looking for a free vnd. locore64 said that vnd
is now a cloning pseudo-device; is that accessible at user level, or
will it take device driver changes?
Unmount requires new something-or-other, since the umount command
doesn't seem to look for umount_fstype, and I'd need that (or changes
to umount(8)) to unconfigure the vnd. Conceptually, though, either
I haven't figured out what to do about disk labels.
--Steve Bellovin, http://www.cs.columbia.edu/~smb
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