mount as ro for users, rw for root?

[Tino Schwarze]

On Sun, Dec 14, 2008 at 10:47:22AM -0500, J. Epperson wrote:

> > multiply mounting a NFS export is OK.  Multiply mounting an iSCSI
> > volume (a LUN, seen as a disk to Linux) is really bad, unless you're
> > using a cluster-aware file system on it, such as GFS.
> 
> For my own education, is that the case when one mount is read only, as was
> suggested?  I guess the ro mount would still be updating atime, so I'm
> thinking that it would be, but if not it could be a useful technique.

RO does not update anything! Thatswhy it's called read-only. The problem
with mounting an FS twice is that writes to the rw mount will probably
not be seen by the second mount - internal caches and kernel structures
which get updated during the write won't get updated for the RO mount ->
things get confused and corrupted. Therefore, mounting the same FS twice
is not supported.

If you try to mount a device a second time, you'll get an error:

# mount -o ro /dev/sda1 to
mount: /dev/sda1 already mounted or to busy
mount: according to mtab, /dev/sda1 is mounted on /mnt/t

Disclaimer: I'm no kernel hacker, that's just my common sense and
understanding of file systems and mounting.

Bye,

Tino.

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