Re: [gentoo-amd64] Soliciting new RAID ideas

[Bob Sanders]

Marc Joliet, mused, then expounded:
> Am Wed, 28 May 2014 08:26:58 -0700
> schrieb Bob Sanders <rsanders@×××.com>:
>
> >
> > Marc Joliet, mused, then expounded:
> > > Am Tue, 27 May 2014 15:39:38 -0700
> > > schrieb Bob Sanders <rsanders@×××.com>:
> > >
> > > While I am far from a filesystem/storage expert (I see myself as a mere user),
> > > the cited threads lead me to believe that this is most likely an
> > > overhyped/misunderstood class of errors (e.g., posts [1] and [2]), so I would
> > > suggest reading them in their entirety.
> > >
> > > [0] http://comments.gmane.org/gmane.comp.file-systems.btrfs/31832
> > > [1] http://permalink.gmane.org/gmane.comp.file-systems.btrfs/31871
> > > [2] http://permalink.gmane.org/gmane.comp.file-systems.btrfs/31877
> > > [3] http://comments.gmane.org/gmane.comp.file-systems.btrfs/31821
> > >
> >
> > FWIW - here's the FreeNAS ZFS ECC discussion on what happens with a bad
> > memory bit and no ECC memory:
> >

Just to beat this dead horse some more, an analysis of a academic study
on drive failures -

http://storagemojo.com/2007/02/20/everything-you-know-about-disks-is-wrong/

And it links to the actual study here -

https://www.usenix.org/legacy/events/fast07/tech/schroeder.html

Which shows that memory has a fairly high failure rate as well, though
the focus is on hard drives.

> > http://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-ram-and-zfs.15449/
>
> Thanks for explicitly linking that.  I didn't read it the first time around,
> but just read through most of it, then reread the threads [0] and [3] above and
> *think* that I understand the problem (and how it doesn't apply to BTRFS)
> better now.
>
> IIUC, the claim is: data is written to disk, but it must go through the RAM
> first, obviously, where it is corrupted (due to a permanent bit flip caused,
> e.g., by deteriorating hardware).  At some later point, when the data is read
> back from disk, it might happen to load around the damaged location in RAM,
> where it is further corrupted.  At this point the checksum fails, and ZFS
> corrects the data in RAM (using parity information!), where it is immediately
> corrupted again (because apparently it is corrected at the same physical
> location in RAM? perhaps this is specific to correction via parity?). This
> *additionally* corrupted data is then written back to disk (without any further
> checks).
>
> So the point is that, apparently, without ECC RAM, you could get a (long-term)
> cascade of errors, especially during a scrub.  The likelihood of such permanent
> RAM corruption happening in the first place is another question entirely.
>
> The various posts in [0] then basically say that regardless of whether this
> really is true of ZFS, it certainly doesn't apply to BTRFS, for various
> reasons.  I suppose this quote from [1] (see above) says it most clearly:
>
> > In hxxp://forums.freenas.org/threads/ecc-vs-non-ecc-ram-and-zfs.15449, they talk about
> > reconstructing corrupted data from parity information:
> >
> > > Ok, no problem. ZFS will check against its parity. Oops, the parity failed since we have a new corrupted
> > bit. Remember, the checksum data was calculated after the corruption from the first memory error
> > occurred. So now the parity data is used to "repair" the bad data. So the data is "fixed" in RAM.
> >
> > i.e. that there is parity information stored with every piece of data, and ZFS will "correct" errors
> > automatically from the parity information.  I start to suspect that there is confusion here between
> > checksumming for data integrity and parity information.  If this is really how ZFS works, then if memory
> > corruption interferes with this process, then I can see how a scrub could be devastating.  I don't know if
> > ZFS really works like this.  It sounds very odd to do this without an additional checksum check.  This sounds
> > very different to what you say below that btrfs does, which is only to check against redundantly-stored
> > copies, which I agree sounds much safer.
>
> The rest is also relevant, but I think the point that the data is corrected via
> parity information, as opposed to using a known-good redundant copy of the data
> (which I originally missed, and thus got confused), is the key point in
> understanding the (supposed) difference in behaviour between ZFS and BTRFS.
>
> All this assumes, of course, that the FreeNAS forum post that ignited this
> discussion is correct in the first place.
>
> > Thanks Mark!  Interesting discussion on btrfs.
> >
> > Bob
>
> You're welcome!  I agree, it's an interesting discussion.  And regarding the
> misspelling of my name: no problem :-) .
>
> --
> Marc Joliet
> --
> "People who think they know everything really annoy those of us who know we
> don't" - Bjarne Stroustrup



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