I didn't know it couldn't be run on a mounted filesystem and it didn't abort as it should have. Instead it gave random errors and SEGV'ed in the end legolas:/mnt/btrfs_pool2# btrfs-image -c 9 -t 6 /dev/mapper/disk2 /tmp/pool2.image parent transid verify failed on 295965446144 wanted 51493 found 51495 parent transid verify failed on 295965446144 wanted 51493 found 51495 parent transid verify failed on 295965446144 wanted 51493 found 51495 parent transid verify failed on 295965446144 wanted 51493 found 51495 Ignoring transid failure leaf parent key incorrect 295965446144 parent transid verify failed on 106205184 wanted 51468 found 51528 parent transid verify failed on 106205184 wanted 51468 found 51528 parent transid verify failed on 106205184 wanted 51468 found 51528 parent transid verify failed on 106205184 wanted 51468 found 51528 Ignoring transid failure parent transid verify failed on 267162120192 wanted 51469 found 51528 parent transid verify failed on 267162120192 wanted 51469 found 51528 parent transid verify failed on 267162120192 wanted 51469 found 51528 parent transid verify failed on 267162120192 wanted 51469 found 51528 Ignoring transid failure parent transid verify failed on 107315200 wanted 51468 found 51528 parent transid verify failed on 107315200 wanted 51468 found 51528 parent transid verify failed on 107315200 wanted 51468 found 51528 parent transid verify failed on 107315200 wanted 51468 found 51528 Ignoring transid failure parent transid verify failed on 108081152 wanted 51468 found 51528 parent transid verify failed on 108081152 wanted 51468 found 51528 parent transid verify failed on 108081152 wanted 51468 found 51528 parent transid verify failed on 108081152 wanted 51468 found 51528 Ignoring transid failure parent transid verify failed on 267162124288 wanted 51469 found 51528 parent transid verify failed on 267162124288 wanted 51469 found 51528 parent transid verify failed on 267162124288 wanted 51469 found 51528 parent transid verify failed on 267162124288 wanted 51469 found 51528 Ignoring transid failure parent transid verify failed on 108351488 wanted 51468 found 51528 parent transid verify failed on 108351488 wanted 51468 found 51528 parent transid verify failed on 108351488 wanted 51468 found 51528 parent transid verify failed on 108351488 wanted 51468 found 51528 Ignoring transid failure parent transid verify failed on 275021406208 wanted 51469 found 51529 parent transid verify failed on 275021406208 wanted 51469 found 51529 parent transid verify failed on 275021406208 wanted 51469 found 51529 parent transid verify failed on 275021406208 wanted 51469 found 51529 Ignoring transid failure parent transid verify failed on 108466176 wanted 51468 found 51528 parent transid verify failed on 108466176 wanted 51468 found 51528 parent transid verify failed on 108466176 wanted 51468 found 51528 parent transid verify failed on 108466176 wanted 51468 found 51528 Ignoring transid failure Segmentation fault
Right, the mount status should be checked. It should be possible to do image from a read-only mounted fs.