$ dd if=/dev/zero of=/tmp/20Gfile bs=1G count=20
20+0 records in
20+0 records out
21474836480 bytes (21 GB) copied, 167.761 s, 128 MB/s
$ mkfs.ext2 /tmp/20Gfile
mke2fs 1.42.9 (4-Feb-2014)
/tmp/20Gfile is not a block special device.
Proceed anyway? (y,n) y
Discarding device blocks: done
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
1310720 inodes, 5242880 blocks
262144 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
160 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
        4096000

Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

$ sudo mount -o loop /tmp/20Gfile /mnt/loop1
$ df -h /mnt/loop1
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0       20G   44M   19G   1% /mnt/loop1