Re: file system with no gratuitous writes?

[Jean-Yves Migeon <jeanyves.migeon%free.fr@localhost>]

On 02.01.2011 22:42, Steven Bellovin wrote:
> I'm building a compact flash-based system for a Soekris box.
> I'd like to set it up so that there are no write operations that
> are not specifically initiated by the administrator.  That is, if

> I modify a configuration file, of course that should be flushed to
> disk; however, I don't want writes at any other time, such as
> rewriting the superblock every 30 seconds.  Is there some way I can

> do it?
> In an older instance of this system, the disk was mounted read-only;
> I'd mount it read-write to updated it, flush the buffer cache, then
> remount it read-only.  On 5.0 and later, however (and as has been
> discussed ad nauseum), that is no longer supported.  The next
> possibility is to have some way to mount the file system so there are
> no such writes.  Specifying noatime will certainly help -- but is that
> enough?  (Log files and such use tmpfs, so they're not a problem.)

> I've also thought about mounting /var/db as a FAT file system -- would
> that do the trick?  Any other suggestions?

Apart mounting /dev, /etc, and /var via mfs mounts (I use this for NFS
root read-only mounts, and works quite nicely), one possibility I can
think of (albeit very impractical) would be to select the file systems
you want to be "administratively writable at a specific time", mirror
them in hierarchy that you only know of, and mount_null it in the right
place.

For example:

/null_mounts/usr/
=>
$ mount_null -o ro /null_mounts/usr/ /usr/

Makes it read only, but you can still modify the files if you do it via
/null_mounts, in case of updates.

Yeah, "ugly".

Happy new year!

-- 
Jean-Yves Migeon
jeanyves.migeon%free.fr@localhost