Problems with dump and restore

[Roland Smith]

On Tue, Aug 12, 2014 at 05:14:56PM -0300, Andrew Hamilton-Wright wrote:
> 
> Sorry, a bit of a follow up after some reading, as the soft-update
> bit was itching in my memory...
> 
> 
> On Tue, 12 Aug 2014, Roland Smith wrote:
> 
> > In 2011, a problem was found with snapshots in combination with soft
> > updates *and* journaling (SU+J) hanging the machine. At that time the
> > recommendation was to switch off journaling.
> > According to https://wiki.freebsd.org/NewFAQs:
> >
> >    If you want to use snapshot (dump -L) then disable the soft updates
> >    journal for that filesystem.
> 
> I have just realize that the soft update journal (newfs option -j enable)
> is actually not in play here, however "soft updates" (option -U) is.  My
> understanding of the referenced bug:
>  	https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=160662
> is that it is specifically referred to _journaled_ soft updates as being
> a danger, but soft updates using -U are not referred to by this bug.
> 
> Is that your understanding as well?

Yes. If you look at the mailing lists traffic around that time you'll see that
it was the *combination* of SU+J that had problems.

> I believe that when I set up the filesystems for this machine in the first
> place, I specifically ensured that "-j disable" was chosen, for exactly
> this reason -- however my logbook does not contain enough detail for
> me to be sure.

Run `mount` and you'll see which options are set on your mounted filesystems;

    /dev/ada0p2 on / (ufs, local, noatime, journaled soft-updates)

> To be clear then, do I understand that your filesystem options include
> neither "-j enable" nor "-U"?

No, I use SU+J. But I do dumps from single user mode on unmounted or read-only
filesystems so snapshots aren't used. And every month or so I run a *full*
fsck before making the dumps.

These measures might be considered an overabundance of caution on my part.
But I've actually lost a couple harddisks due to hardware failure over the
years. And since it's little effort I'd rather be safe than sorry.

Roland
-- 
R.F.Smith                                   http://rsmith.home.xs4all.nl/
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