Re: Remounting as read-only

[Steven Bellovin <smb%cs.columbia.edu@localhost>]

On May 14, 2010, at 11:18 32AM, Thor Lancelot Simon wrote:

> On Fri, May 14, 2010 at 01:27:08PM +0000, Wouter Klouwen wrote:
>> Emmanuel Dreyfus(manu%netbsd.org@localhost) said 2010.05.14 13:13:47 +0000:
>>> On Fri, May 14, 2010 at 01:00:53PM +0000, Emmanuel Dreyfus wrote:
>>>> Of course I acan umount and remount, but that invalidate file descriptors
>>>> that had been open read-only on the filesystem. Isthere any trick to get
>>>> it working as it worked on NetBSD 4?
>>> 
>>> Ok, I found it: mount -f -u /flash
>> 
>> That updates it to rw, which is not what you want. There was another person
>> asking for this in NetBSD 5.0. Turns out it can't sync meta data upon
>> remount read only: http://gnats.netbsd.org/30525
> 
> I'm not sure what you mean by "it", but I should point out that the bug
> in question -- kernel doesn't sync metadata on read-only remount -- is
> extremely old, and *did* cause disk corruption after r/o remount set
> the filesystem "clean" flag, thus corruption fsck would never find and
> which could potentially eat your disk over time.
> 
> It's *always* been broken.  Not just in NetBSD, probably in 4.4BSD
> and prior kernels as well.  It is unquestionably broken at least in older
> releases of the other BSDs as well. We have opted for safety over a false
> sense of security.

Yup, understood.  I just want it to work, and work properly; it's a useful 
feature for embedded systems:

        # uptime
        11:21AM up 593 days, 14:32, 0 users, load averages: 0.06, 0.08, 0.08 

With, probably, half a dozen configuration changes during that time.

                --Steve Bellovin, http://www.cs.columbia.edu/~smb