On 7/3/20 10:39 PM, Eric Sandeen 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.
I ran 50 loops, and got:
46 btrfsck failures
20 mount failures
So it ran btrfs restore 20 times; of those, 11 runs lost all or
substantially all of the files; 17 runs lost at least 1/3 of the
files.
Hmm I wonder if some of my "ignore X failures" stuff got lost over the years, we
should be able to recover far more than that. I'll add it to the list of things
to dig into this week. Thanks,
Josef
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