Сделал образ xfs внутри ext4 размером 30 гигов методом dd if=/dev/zero of=myxfs. В итоге при попытке mkfs.xfs получил кучу сообщений в dmesg'е:
[ 7330.727873] EXT4-fs error (device sda2): ext4_ext_map_blocks:3771: inode #5931034: comm loop1: bad extent address lblock: 10485728, depth: 2 pblock 0
[ 7330.728171] EXT4-fs error (device sda2): ext4_ext_map_blocks:3771: inode #5931034: comm loop1: bad extent address lblock: 10485728, depth: 2 pblock 0
[ 7330.728475] EXT4-fs error (device sda2): ext4_ext_map_blocks:3771: inode #5931034: comm loop1: bad extent address lblock: 10485729, depth: 2 pblock 0
[ 7330.728780] EXT4-fs error (device sda2): ext4_ext_map_blocks:3771: inode #5931034: comm loop1: bad extent address lblock: 10485728, depth: 2 pblock 0
[ 7330.729090] EXT4-fs error (device sda2): ext4_ext_map_blocks:3771: inode #5931034: comm loop1: bad extent address lblock: 10485729, depth: 2 pblock 0