UFS Crash and directories now missing

[Alejandro Imass]

On Sat, Apr 28, 2012 at 2:01 PM, Polytropon <freebsd at edvax.de> wrote:
> On Sat, 28 Apr 2012 13:52:02 -0400, Alejandro Imass wrote:
>> On Sat, Apr 28, 2012 at 1:31 PM, Robert Bonomi <bonomi at mail.r-bonomi.com> wrote:
>> >
>> > Alejandro Imass <aimass at yabarana.com> wrote:
>> >> On Sat, Apr 28, 2012 at 11:39 AM, Robert Bonomi
>> >> <bonomi at mail.r-bonomi.com> wrote:
>> >> >  Alejandro Imass <aimass at yabarana.com> wrote:
>> >> >> After a little more research, ___it it NOT unlikely at all___ that
>> >> >> under high distress and a hard boot, UFS could have somehow corrupted
>> >> >> the directory structure, whilst maintaining the data intact.
>> >> >
>> >> > This is techically accurate, *BUT* the specifics of the quote "corruption"
>> >> > unquote in the case under discussion make it *EXTREMELY* unlikely that this
>> >> > is what happened.
>> >> >
>> >> > 99.99+++% of all UFS filesystem "corruption' issues are the result of a
>> >> > system crash _between_ the time cached 'meta-data' is updated in memory
>> >> > and that data is flushed to disk (a deferred write).
>> >> >
>> >> > The second most common (and vanishingly rare) failure mode is a powerfail
>> >> > _as_ a sector of disk is being written -- resulting in 'garbage data'
>> >> > being written to disk.
>> >> >
>> >> > The next possibility is 'cosmic rays'.  If running on 'cheap' hardware
>> >> > (i.e., without 'ECC' memory), this can cause a *SINGLE-BIT* error in
>> >> > data being output.
>> >> >
>> >> > The fact that the 'corrupted' filesystem passed fsck -without- any reported
>> >> > errors shows that everything in the filesystem meta-data was consistent
>> >> >
>> >> [...]
>> >>
>> >> > I think it is safe to conclude that the probabilities -greatly- favor
>> >> > alternative #1.
>> >> >
>> >>
>> >> OK. So after your comments and further research I concur with you on
>> >> the mv but if it wasn't a human, then this might be exposing a serious
>> >> security flaw in the jail system or the way EzJail implements it.
>> >
>> > BOGON ALERT!!!
>> >
>>
>> I admit my ignorance on how the filesystem works but I don't think
>> your condescending remarks add a lot of value. The issue here is this
>> actually happened and there is a flaw somewhere other than "the stupid
>> administrator did it".
>
> If you search the archives of this list, you'll find my _first_
> post to that list: I've had a similar problem, df shows data
> must be there after crash (panic -> reboot -> fsck trouble), but
> files aren't there (even _not_ in lost+found). It's quite possible
> that in _exceptional_ moments this can happen. The fsck program
> is intended to repair the most typical file system faults, but
> nothing "complicated" will be done without interaction: Altering
> data on disk will _always_ involve the responsible (!) admin to
> check if it is really intended "to do so".
>
> There can be many reasons. I've never found out what was the
[...]

> that might help locate "lost" data (quotes intended as long as
> the data is still on the disk). The more complex your setting
> is (e. g. striped disks, or ZFS), this can be nearly impossible.
> "Plain old UFS" can sometimes be your saviour (but BACKUP should
> be your real friend).
>

Thanks for your reply.

I can't figure out how there was no data loss and yet the directories
moved just like that. We have nightly backups and it's one of the
features we love about EzJail and it's archive feature. The base
system sits on another disk entirely and it's pristine, we don't
install anything except the basic system on the system disk and the
other disk is exclusively divided in jails, so the possibility of an
outside process doing the mv is unlikely.

Everything point to that something or someone executed a mv but how
was this done? or if there is a potential problem and could happen
again. And contrary to other comments here, and my admitted ignorance,
I believe there are actually 3 possibilities:

1) something inside a jail was able to move the other jails into itself
2) something outside the jails moved the jails
3) the directories were moved at reboot by journal recovery, fsck or
something else

That is what worries me, is that it wasn't just some random bit or
cosmic ray, but the potential of happening again. I am not so sure
that it is *impossible* that a jail could affect other jails with
EzJail.

-- 
Alejandro