Create large files

With dd (fast, full files, controlled content (ex: zeroes))

dd if=/dev/zero of=big_file.img bs=1 count=0 seek=10G

With truncate (very fast, sparse files, no content, requires sparse file system support)

truncate -s 10G big_file.img

With fallocate (fast, full files, no content)

fallocate -l 10G big_file.img