Re: Fixing a corrupted file system

[Amit Shah <amit@infradead.org>]

Hello Qu,


On Thu, 2026-04-02 at 07:51 +1030, Qu Wenruo wrote:
> 
> 
> 在 2026/4/2 02:28, Amit Shah 写道:
> > Hello folks,
> > 
> > I had a msata drive that started showing signs of failure - error
> > reading sectors.  I had a single LUKS partition on it (for
> > encryption)
> > with btrfs as the file system taking up the entire space (1 TB).
> > 
> > I got all the raw data from the drive via ddrescue, and have dumped
> > it
> > into a second drive.  However, it looks like before I noticed the
> > disk
> > errors, btrfs tried correcting them based on the erroneous reads on
> > the
> > first disk, causing those errors to propagate on the recovered
> > copy:
> 
> Nope, btrfs will only try extra mirrors, but in your case your
> metadata 
> doesn't even have extra mirrors.
> Which is not common, as the default btrfs mkfs profiles will go DUP
> for 
> metadata.

Hm, looks like the functionality for auto-enabling metadata DUP only
landed in 5.15, and perhaps my install was from before that?  I didn't
go out of my way to select non-default entries.

> So you only have one chance and lost that chance.

[...]

> > 1. Can I do anything to fix the metadata for this filesystem, and
> > have
> > it mount, recognize and recover the data on there?
> 
> Not really. You only have one copy or metadata, and it's corrupted.
> 
> It may be possible to repair, but it's definitely not reliable and I 
> won't recommend to do it.
> 
> All you can do is try to salvage the data, by mounting with 
> "rescue=all,ro" which should give you some chance to read out some
> data.

Oh this is very useful, thanks.  I tried btrfs-rescue as a separate
command and only could reach a handful files.  Using it as a mount
option does give me a more complete-looking directory tree. As
expected, a few files have errors reading them, but most of them do
open fine.

> > 2. Does it make sense to drop down the filesystem to read-only when
> > the
> > underlying drive starts exhibiting errors?
> 
> For your case, it makes no difference.

Oh sure.  This isn't a question to fix anything for me now.  This is
just a way of throwing out a suggestion for a future feature - whether
something like that makes sense.

> Your metadata is only SINGLE, and that's your choice, so there is no 
> repair anyway.
> 
> Furthermore even with extra mirrors (e.g. DUP or RAID1), btrfs will
> only 
> write the correct data/metadata into the bad one, thus it should not 
> make things worse unless your hardware is totally screwed up.

Yea, that's understandable, of course.

Thanks a lot for the mount option hint, it has already helped me poke
around the file system.

		Amit