Failed ReadyNas 104, mount BTRFS volume on Linux

Here is root cause and  solution I found:

incompat_flags 0x21
( MIXED_BACKREF |
BIG_METADATA )

It seems my ReadyNas migrated to btrfs at some strange point, where those flags where enabled for btrfs, this resulted btrfs being incompartible with btrfs in ubuntu (it was 5.x version).

Is it an issue for all ReadyNas with i.e. ReadyNas OS have those flags for everynas or is it dependant on time of migration to btrfs, it's hard to say. I was running quite latest version of OS available at that moment.

So, eventually I solved the problem using Ubuntu 14, it just worked with btrfs - no incompatibliyt issues. Could it work on something between Ubuntu 14 and Ubuntu 20, - i don't know. But you may want to experiment if you faced this issue.

As there is coninuation of story.

Once I mounted btrfs I had to restore several LUNs, several of them where encrypted with VeraCrypt.

There are multiple guides to mount iscsi file via loop device, e.g. here http://infotinks.com/mount-luns-with-partitions-using-losetup-and-kpartx/

where you would use command

# losetup /dev/loop0 lunfile -o OFFSET

To calculate offset you need to use command and multilply offset by sector size normally 512.

# fdisk -lu lunfile

The problem with Ubuntu 14, fdisk returns wrong results and does not show correct offset.

# sgdisk -p lunfile - does not work either, despite fdisk show partition in lunfile as GPT.

So, i had to copy lunfile to another linux and run fdisk on lunfile there and it worked ok gave me expected rusult of offset (I can't remember exact offset which was used and I did not keep lun files).

Then I was able to mount lunfile via losetup as expected.

On encrypted lunfiles, i tried to expose on another system via iSCSI hoping to mount them via VeraCrypt on windows machine, but probably iSCSI demon implementation also changed and another linux could not expose raw devices properly.

So, i downlowed quite old console version of VeraCrypt to match Ubunu 14 and after mounting lunpartion via loopback was able to mount it via console version of VeraCrypt.

So, it was happy end for me. Restoring data from ReadyNas is possible, but good guide or better live linux distro would be nice.

I think it's not diffucult to build linux distro with compatible versions of mdadm, btrfs, fdisk, iSCSI, so, if someone face similar issue again - you just plug your disks and live drive into PC and follow instuction...