Hi all, my btrfs root file system on a SD card broke down and did not mount anymore.
In retrospective, I think it reached its endurance, so I know that there is nothing to repair. All I want to do is to salvage some configuration and data files from the remains left in my ISO file copy. The SD card is no longer readable, so all I have is the 30GB "dd" copy of the btrfs partition. I also tried some things on the ISO file I later found I shouldn't have done with the "btrfs" tools, which I think broke the file system in it even more. So at this stage, this is the "dmesg" output when trying to mount the ISO file, which then fails: [ 249.239883] BTRFS: device fsid 4235aa4f-7721-4e73-97f0-7fe0e9a3ce9c devid 1 transid 1757933 /dev/loop2 [ 249.241504] BTRFS info (device loop2): disk space caching is enabled [ 249.275950] BTRFS error (device loop2): bad tree block start 0 20987904 [ 249.280936] BTRFS error (device loop2): bad tree block start 0 20987904 [ 249.280946] BTRFS error (device loop2): failed to read chunk root [ 249.336291] BTRFS error (device loop2): open_ctree failed Output of "uname -a": Linux desinfect 4.13.0-39-generic #44~16.04.1-Ubuntu SMP Thu Apr 5 16:43:10 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux Output of "btrfs --version": btrfs-progs v4.4 When reading the ISO file with "Active@ Disk Editor" (a hex file editor) I find a super block at offset 0x10000 that looks like this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he super block at offset 0x4000000 is zeroed out. When looking at the addresses of chunk root (0x1404000), root of tree root (0x34FF4000) and log tree root (0x350E0000) in the first super block they are all zeroed out as well. So I think I understand why the error "failed to read chunk root" crops up. If I try to "restore" using "btrfs restore sdcard.iso /outdir" I get this output: checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 bytenr mismatch, want=20987904, have=0 Couldn't read chunk root Could not open root, trying backup super No valid Btrfs found on sdcard.iso Could not open root, trying backup super Superblock bytenr is larger than device size Could not open root, trying backup super And, finally, I can see "/etc" someplace near "fstab" in the ISO which looks like a directory listing as well as content of files I remember, which tells me, that the data I still in there somewhere. So, what can I do to get the files I need out of this blob. I am willing to follow data pointers as described in https://btrfs.wiki.kernel.org/index.php/On-disk_Format in the hex editor and copy the data from there. Can anyone give me any pointers into the ISO file (maybe starting from the super block) to help me extract the data I need? Cheers, Mirko
