lvcreate -L 2G -n swap vg_virt
  Logical volume "swap" created
mkswap /dev/vg_virt/swap 
mkswap: /dev/vg_virt/swap: warning: don't erase bootbits sectors
        on whole disk. Use -f to force.
Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=2cbaf464-5aee-45b0-9909-0e1bc5e3d3ce
swapon /dev/vg_virt/swap