Re: [gentoo-dev] rfc: Does OpenRC really need mount-ro

[Richard Yao]

> On Feb 16, 2016, at 1:41 PM, William Hubbs <williamh@g.o> wrote:
>
>> On Tue, Feb 16, 2016 at 01:22:13PM -0500, Rich Freeman wrote:
>>> On Tue, Feb 16, 2016 at 1:05 PM, William Hubbs <williamh@g.o> wrote:
>>>
>>> The reason it exists is very vague to me; I think it has something to do
>>> with claims of data loss in the past.
>>
>> Is there some other event that will cause all filesystems to be
>> remounted read-only or unmounted before shutdown?
>
> When localmount/netmount stop they try to unmount file systems they know
> about, but they do not try to remount anything.
>
>
>> You definitely will want to either unmount or remount readonly all
>> filesystems prior to rebooting.  I don't think the kernel guarantees
>> that this will happen (I'd have to look at it).  Just doing a sync
>> before poweroff doesn't seem ideal - if nothing else it will leave
>> filesystems marked as dirty and likely force fscks on the next boot
>> (or at least it should - if it doesn't that is another opportunity for
>> data loss).
>>
>> There are different ways of accomplishing this of course, but you
>> really want to have everything read-only in the end.
>
> unmounting is easy enough; we already do that.
>
> What I'm trying to figure out is, what to do about re-mounting file
> systems read-only.
>
> How does systemd do this? I didn't find an equivalent of the mount-ro
> service there.

One idea proposed by systemd that is almost never used in production is to fall back to an initramfs environment to undo the boot process by umounting /. It would not surprise me if the normal case were hard coded to remount / as ro because you risk filesystem corruption otherwise. Journaling filesystems are fairly good at surviving that, but you are still taking a risk due to partial writes and anyone using ext2 would be taking a much bigger gamble.
>
> William