Re: Reading files with bad data checksum

[Goffredo Baroncelli <kreijack@libero.it>]

On 1/10/21 1:36 PM, David Woodhouse wrote:
> On Sun, 2021-01-10 at 13:08 +0100, Forza wrote:
[...]
> 
> It showed up as errors. There appears to be a btrfs bug there but since

Yes, it is an old btrfs bug. And qemu is not the guilty.

https://lore.kernel.org/linux-btrfs/cf8a733f-2c9d-7ffe-e865-4c13d99dfb60@libero.it/

In my email there is a code to reproduce it.

Basically it is difficult to have the checksum sync with the data when O_DIRECT
is used. Even OpenZFS has problem with it. The OpenZFS solution is to lie about O_DIRECT:
it allows the flag however it doesn't honor.

I think that BTRFS should behave like ZFS: when csum are enable, O_DIRECT shouldn't be
honored (or returning an error or behaving like ZFS).


> I suspect it'll be easy to reproduce I'm more focused on recovery right
> now.
> 
>>> In the short term, all I want to do is make a copy of the file, using
>>> the data which are in the disk regardless of the fact that btrfs thinks
>>> the checksum doesn't match. Is there a way I can turn off *checking* of
>>> the checksum for that specific file (or file descriptor?).
>>>
>>> Or is the only way to do it with something like FIBMAP, reading the
>>> offending blocks directly from the underlying disk and then writing
>>> them into the appropriate offset in (a copy of) the file? A plan which
>>> is slightly complicated by the fact that of course btrfs doesn't
>>> support FIBMAP.
>>>
>>> What's the best way to recover the data?
>>>
>>
>> You can use GNU ddrescue to copy files. It can skip the offending blocks
>> and replace the bad data with zeroes. Not sure how well qemu will handle
>> that though.
> 
> Right. I've already copied the image with dd conv=sync,noerror to a new
> one with the +C flag. It passes 'qemu-img check', and in fact the guest
> is running just fine with it. I was expecting it to stop with
> catastrophic file system errors but I can't see anything wrong at all.
> I'm just paranoid that eventually I'll find out that the blocks belong
> to some file(s) I actually want, and I'd like to recover them.
> 
> Right now I have a horribly fragmented image file with these 'errors'
> cluttering up my file system and making backups of the host go
> extremely slow. I'd like to get those blocks back so I can make a clean
> copy of the image, and keep it around for reference in case I later
> *do* discover that I need the contents of those blocks.
> 


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