Ext4 to Btrfs Conversion – Fixing ‘Missing Data Block’ Error

arch linuxbtrfsext4filesystems

I am on a test VM where I am trying to convert a second disk to btrfs.

The conversion fails with the error missing data block for bytenr 1048576 (see below).

I couldn't find any information about the error. What can I do to fix this?

$ fsck -f /dev/sdb1
fsck from util-linux 2.35.2
e2fsck 1.45.6 (20-Mar-2020)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/sdb1: 150510/4194304 files (0.5% non-contiguous), 2726652/16777216 blocks

$ btrfs-convert /dev/sdb1
create btrfs filesystem:
        blocksize: 4096
        nodesize:  16384
        features:  extref, skinny-metadata (default)
        checksum:  crc32c
creating ext2 image file
ERROR: missing data block for bytenr 1048576
ERROR: failed to create ext2_saved/image: -2
WARNING: an error occurred during conversion, filesystem is partially created but not finalized and not mountable

Best Answer

It was a bug

Now we have pinned down the bug, it's a bit overflow for multiplying unsigned int.

Also see:

As a workaround you can resize your filesystem to something smaller/larger before the conversion.

Related Solutions

Linux – Should a laptop user switch from ext4 to btrfs

I agree with vonbrand, btrfs is not yet to the maturity level of ext* or XFS or JFS to name a few. I would not use it on a laptop with precious data unless I have a reliable backup that can be done also on the go.

Btrfs can detect corruptions but it won't do anything more than reporting the detection unless you have an available uncorrupted copy of the same data, which means you either need RAID or duplication of data on the volume.

That said, I am considering using it (using RAID-1) for one machine, but I also do have Crashplan running on this machine!

For a long time, I have been using JFS on my laptop. One reason was the lower CPU usage compare to XFS or ext3 when doing file operations. I have never verified if it saved power consumption as well, but that was my assumption. I found JFS pretty stable and safe, never lost data while using it.

Ext4 to btrfs conversion: If I accidentally lose power to the system during the conversion, will I lose the data

The data loose most possible due controller write operation failure to disk, while its possible that whole disk partition table would be corrupted.

As it was said, make a backup prior, but the conversion operation it self is designed so, that NO ORIGINAL DATA is touched. Btrfs makes snapshot of original FS version and uses free blocks to make further operations.

If you will create fresh ext4 FS on 100GB partition and run

#df -h
Filesystem                        Size  Used Avail Use% Mounted on
/dev/sda3                          99G   60M   94G   1% /ext4convert2btrfs

then unmount and run conversion:

#unmount /ext4convert2btrfs
#btrfs-convert /dev/sda3
creating btrfs metadata.
creating ext2fs image file.
cleaning up system chunk.
conversion complete.

then mount run df again, and you will see something like this:

Filesystem                        Size  Used Avail Use% Mounted on
/dev/sda3                         100G  1.8G   66G   3% /ext4convert2btrfs

The rest of space is used by primary FS snapshot. Only system administrator will decide when it is safe to remove snapshot and allow original data to be overwritten.

To recover free space and make the conversion permanent, do:

#cd /ext4convert2btrfs
#btrfs subvol delete ext2_saved
#btrfs balance start /ext4convert2btrfs
#df -h
Filesystem                        Size  Used Avail Use% Mounted on
/dev/sda3                         100G  544K   99G   1% /ext4convert2btrfs

That's it :)

Best Answer

Related Solutions

Linux – Should a laptop user switch from ext4 to btrfs

Ext4 to btrfs conversion: If I accidentally lose power to the system during the conversion, will I lose the data

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