Create a sparse file for the fs using fallocate. As per the man page:
“fallocate is used to preallocate blocks to a file. For filesystems which support the fallocate system call, this is done quickly by allocating blocks and marking them as uninitialized, requiring no IO to the data blocks. This is much faster than creating a file by filling it with zeros.”
[ed@orinoco lfs]$ fallocate -l 20G root.image
Create the filesystem:
[ed@orinoco lfs]$ mkfs.ext4 -F root.image
Mount the file via loop
[ed@orinoco lfs]$ sudo mount root.image ./work/root -o loop