On Tue, Apr 30, 2019 at 8:05 AM Michelle Sullivan <michelle at sorbs.net>
wrote:>
>
>
> Michelle Sullivan
> http://www.mhix.org/
> Sent from my iPad
>
> > On 01 May 2019, at 00:01, Alan Somers <asomers at freebsd.org>
wrote:
> >
> >> On Tue, Apr 30, 2019 at 7:30 AM Michelle Sullivan <michelle at
sorbs.net> wrote:
> >>
> >> Karl Denninger wrote:
> >>> On 4/30/2019 05:14, Michelle Sullivan wrote:
> >>>>>> On 30 Apr 2019, at 19:50, Xin LI <delphij at
gmail.com> wrote:
> >>>>>> On Tue, Apr 30, 2019 at 5:08 PM Michelle Sullivan
<michelle at sorbs.net> wrote:
> >>>>>> but in my recent experience 2 issues colliding at
the same time results in disaster
> >>>>> Do we know exactly what kind of corruption happen to
your pool? If you see it twice in a row, it might suggest a software bug that
should be investigated.
> >>>>>
> >>>>> All I know is it?s a checksum error on a meta slab
(122) and from what I can gather it?s the spacemap that is corrupt... but I am
no expert. I don?t believe it?s a software fault as such, because this was
cause by a hard outage (damaged UPSes) whilst resilvering a single (but
completely failed) drive. ...and after the first outage a second occurred (same
as the first but more damaging to the power hardware)... the host itself was not
damaged nor were the drives or controller.
> >>> .....
> >>>>> Note that ZFS stores multiple copies of its essential
metadata, and in my experience with my old, consumer grade crappy hardware
(non-ECC RAM, with several faulty, single hard drive pool: bad enough to crash
almost monthly and damages my data from time to time),
> >>>> This was a top end consumer grade mb with non ecc ram that
had been running for 8+ years without fault (except for hard drive platter
failures.). Uptime would have been years if it wasn?t for patching.
> >>> Yuck.
> >>>
> >>> I'm sorry, but that may well be what nailed you.
> >>>
> >>> ECC is not just about the random cosmic ray. It also saves
your bacon
> >>> when there are power glitches.
> >>
> >> No. Sorry no. If the data is only half to disk, ECC isn't
going to save
> >> you at all... it's all about power on the drives to complete
the write.
> >
> > ECC RAM isn't about saving the last few seconds' worth of data
from
> > before a power crash. It's about not corrupting the data that
gets
> > written long before a crash. If you have non-ECC RAM, then a cosmic
> > ray/alpha ray/row hammer attack/bad luck can corrupt data after
it's
> > been checksummed but before it gets DMAed to disk. Then disk will
> > contain corrupt data and you won't know it until you try to read
it
> > back.
>
> I know this... unless I misread Karl?s message he implied the ECC would
have saved the corruption in the crash... which is patently false... I think
you?ll agree..
I don't think that's what Karl meant. I think he meant that the
non-ECC RAM could've caused latent corruption that was only detected
when the crash forced a reboot and resilver.
>
> Michelle
>
>
> >
> > -Alan
> >
> >>>
> >>> Unfortunately however there is also cache memory on most
modern hard
> >>> drives, most of the time (unless you explicitly shut it off)
it's on for
> >>> write caching, and it'll nail you too. Oh, and it's
never, in my
> >>> experience, ECC.
> >
> > Fortunately, ZFS never sends non-checksummed data to the hard drive.
> > So an error in the hard drive's cache ram will usually get
detected by
> > the ZFS checksum.
> >
> >>
> >> No comment on that - you're right in the first part, I
can't comment if
> >> there are drives with ECC.
> >>
> >>>
> >>> In addition, however, and this is something I learned a LONG
time ago
> >>> (think Z-80 processors!) is that as in so many very important
things
> >>> "two is one and one is none."
> >>>
> >>> In other words without a backup you WILL lose data eventually,
and it
> >>> WILL be important.
> >>>
> >>> Raidz2 is very nice, but as the name implies it you have two
> >>> redundancies. If you take three errors, or if, God forbid,
you *write*
> >>> a block that has a bad checksum in it because it got scrambled
while in
> >>> RAM, you're dead if that happens in the wrong place.
> >>
> >> Or in my case you write part data therefore invalidating the
checksum...
> >>>
> >>>> Yeah.. unlike UFS that has to get really really hosed to
restore from backup with nothing recoverable it seems ZFS can get hosed where
issues occur in just the wrong bit... but mostly it is recoverable (and my
experience has been some nasty shit that always ended up being recoverable.)
> >>>>
> >>>> Michelle
> >>> Oh that is definitely NOT true.... again, from hard
experience,
> >>> including (but not limited to) on FreeBSD.
> >>>
> >>> My experience is that ZFS is materially more-resilient but
there is no
> >>> such thing as "can never be corrupted by any set of
events."
> >>
> >> The latter part is true - and my blog and my current situation is
not
> >> limited to or aimed at FreeBSD specifically, FreeBSD is my
experience.
> >> The former part... it has been very resilient, but I think (based
on
> >> this certain set of events) it is easily corruptible and I have
just
> >> been lucky. You just have to hit a certain write to activate the
issue,
> >> and whilst that write and issue might be very very difficult
(read: hit
> >> and miss) to hit in normal every day scenarios it can and will
> >> eventually happen.
> >>
> >>> Backup
> >>> strategies for moderately large (e.g. many Terabytes) to very
large
> >>> (e.g. Petabytes and beyond) get quite complex but they're
also very
> >>> necessary.
> >>>
> >> and there in lies the problem. If you don't have a many
10's of
> >> thousands of dollars backup solutions, you're either:
> >>
> >> 1/ down for a looooong time.
> >> 2/ losing all data and starting again...
> >>
> >> ..and that's the problem... ufs you can recover most (in most
> >> situations) and providing the *data* is there uncorrupted by the
fault
> >> you can get it all off with various tools even if it is a complete
> >> mess.... here I am with the data that is apparently ok, but the
> >> metadata is corrupt (and note: as I had stopped writing to the
drive
> >> when it started resilvering the data - all of it - should be
intact...
> >> even if a mess.)
> >>
> >> Michelle
> >>
> >> --
> >> Michelle Sullivan
> >> http://www.mhix.org/
> >>
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