Imagine a situation when a file became corrupted on a single device Btrfs. And in this case file contents are needed as is including corrupted bits.

As stated in man page for mount:

              Enable/disable data checksumming for newly created files.

So it won't disable checksum checks when reading files.

And as I guess all standard system calls involving filesystem for reading will fail at this file. Raw disk read is not an option because of likely fragmentation.

When I try to cat such file, I get error:

cat: file: Input/output error

dmesg reports messages like these:

[631847.884641] BTRFS warning (device loop0): csum failed ino 257 off 0 csum 1280268577 expected csum 2391276770

Having information that corruption occurs at off 0, so this is first 4096 bytes of file, indeed I'm able to retrieve non-corrupted blocks pretty easily:

dd if=file bs=4K skip=1

Btw, writing and appending data to non-corrupted blocks of corrupted file also succeeds. At least when there is only one corrupted block and it's not the last. At first it surprised me, but then I think probably it's a good feature, so that applications that must append critical data won't stuck on a corrupted file.

The question is how do I retrive data from corrupted block (or entire corrupted file)?


As a last resort you can try

btrfs check --init-csum-tree /tmp/copy_of_the_device.bin

This command will change the filesystem and the result can be worse than before, so run this only on a dd or ddrescue copy of the filesystem.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.