Re: Remounting as read-only

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

On May 15, 2010, at 1:37 29AM, jgw%freeshell.org@localhost wrote:

> Stephen Borrill <netbsd%precedence.co.uk@localhost> wrote:
> 
>> On Fri, 14 May 2010, Steven Bellovin wrote:
>>>>> 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.
>> 
>> *nod* That's exactly how I use it too (in thousands of machines in 
>> production daily with no problems). I suppose it might be argued that you 
>> could use WAPBL now.
> 
> I'm curious if there are any "best practices" for those wishing to install
> NetBSD in its current form (ie. WAPBL, ptyfs, kernel modules, ??) on
> compact flash (and similar media).  Most of the guides seem to be several
> years old now and, to a non-developer like myself, it's unclear what tweaks
> should be made.  In the case of WAPBL (journaling) I'd think it ought to
> be turned off to prevent excessive disc writes, no?

The guide that I followed, almost 5 years ago, had an rc.d script that created 
a few mfs or tmpfs file systems as overlay mounts on things like /var; this 
allows the CF disk to be mounted read-only.

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