On Mon, 6 Jul 2020 at 01:19, Chris Murphy <li...@colorremedies.com> wrote:
> On Fri, Jul 3, 2020 at 8:40 PM Eric Sandeen <sand...@redhat.com> wrote:
> >
> > On 7/3/20 1:41 PM, Chris Murphy wrote:
> > > SSDs can fail in weird ways. Some spew garbage as they're failing,
> > > some go read-only. I've seen both. I don't have stats on how common it
> > > is for an SSD to go read-only as it fails, but once it happens you
> > > cannot fsck it. It won't accept writes. If it won't mount, your only
> > > chance to recover data is some kind of offline scrape tool. And Btrfs
> > > does have a very very good scrape tool, in terms of its success rate -
> > > UX is scary. But that can and will improve.
> >
> > Ok, you and Josef have both recommended the btrfs restore ("scrape")
> > tool as a next recovery step after fsck fails, and I figured we should
> > check that out, to see if that alleviates the concerns about
> > recoverability of user data in the face of corruption.
> >
> > I also realized that mkfs of an image isn't representative of an SSD
> > system typical of Fedora laptops, so I added "-m single" to mkfs,
> > because this will be the mkfs.btrfs default on SSDs (right?).  Based
> > on Josef's description of fsck's algorithm of throwing away any
> > block with a bad CRC this seemed worth testing.
> >
> > I also turned fuzzing /down/ to hitting 2048 bytes out of the 1G
> > image, or a bit less than 1% of the filesystem blocks, at random.
> > This is 1/4 the fuzzing rate from the original test.
> >
> > So: -m single, fuzz 2048 bytes of 1G image, run btrfsck --repair,
> > mount, mount w/ recovery, and then restore ("scrape") if all that
> > fails, see what we get.
> What's the probability of this kind of corruption occurring in the
> real world? If the probability is so low it can't practically be
> computed, how do we assess the risk? And if we can't assess risk,
> what's the basis of concern?

Aren't most disk failure tests 'huh it somehow happened at least once
and I think this explains all these other failures too?' I know that
with giant clusters you can do more testing but you also have a lot of
things like

What is the chance that a disk will die over time? 100%
What is the chance that a disk died from this particular scenario?
0.00000<maybe put a digit here> %
reword the question slightly differently.. What is the chance this
disk died from that scenario? 100%.

For the HPC computers we had a score of Phd staticians coming up with
all kinds of papers on disk failure modes which if asked in one way
would come up with practically 0% odds it would happen. However all of
the disk failures had happened at least once over a time frame...
sometimes a short one, sometimes a long one, sometimes so often that
someone had to retract a paper because it was clear that while the
maths said it shouldn't happen .. it did in real life. <welcome to HPC
at high altitudes.. cosmic rays, low air pressure, and dry air need to
be factored in>

Stephen J Smoogen.
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