Re: About /etc/mtab and /proc/mounts

[jw schultz <jw@pegasys.ws>]

On Thu, Feb 27, 2003 at 05:42:30PM +0900, Miles Bader wrote:
> Kasper Dupont <kasperd@daimi.au.dk> writes:
> > > Yes.  On some systems, /var and /tmp are the _only_ read-write filesystems.
> > 
> > OK, but then on such a system with my approach it would be possible to
> > make /mtab.d a symlink pointing to somewhere under /var.
> 
> ... you could do the same with /etc/mtab.
> 
> In fact since /etc is almost guaranteed to be on the same filesystem as
> /, it seems like "/mtab.d" offers zero advantages over just /etc/mtab --
> the case where /etc/mtab is the most annoying is when /etc is R/O, but
> this almost always means that / will be R/O, making /mtab.d useless too.

If you netboot /etc as its own filesystem make sense.  Why
duplicate the rest of root just for /etc.  /etc, /var and
/tmp are the only filesystems that have much reason to be
unique to a system; all others are easily sharable and most
others read-only.

> 
> > But AFAIK fsck uses mtab.
> 
> It uses /etc/fstab.
> 
> > If mtab does not exist mount will attempt to create a new one with
> > only the root listed.
> 
> Unless you use the `-n' flag, which an init-script should do if it
> knows there's something wierd required to get /var mounted or something.

<voice of annoyance>
/etc/mtab is a hack.  I suspect it was done so that fsck, df
and umount wouldn't have to read /dev/kmem.  We now have
much better ways to get data out of the kernel.

The idea of storing the list of mounted filesystems on a
mounted filesystem is a bad idea from the get-go.  The only
advantage it ever really had was the possibility to manually
play mountpoint monte with mv.  Duplicating the in-kernel
data externally begs for inconsistencies that only get worse
with pivot root.

Let the data reside in the kernel and have a procfs or sysfs
entity for it.  A symlink from /etc/mtab can keep the old
tools happy.

-- 
________________________________________________________________
	J.W. Schultz            Pegasystems Technologies
	email address:		jw@pegasys.ws

		Remember Cernan and Schmitt