[root@marek ~]# dd if=/dev/zero of=/swap bs=100M count=20
20+0 records in
20+0 records out
2097152000 bytes (2.1 GB) copied, 21.0326 s, 99.7 MB/s
[root@marek ~]# mkswap /swap 
mkswap: /swap: warning: don't erase bootbits sectors
        on whole disk. Use -f to force.
Setting up swapspace version 1, size = 2047996 KiB
no label, UUID=66cb4fcd-080a-4beb-a068-9eb7c0e8b7fe
[root@marek ~]# swapon /swap