root:~# lvcreate --size 512M --name swap mainvg
  Logical volume "swap" created
root:~# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/mainvg/root
  VG Name                mainvg
  LV UUID                TsqTw4-Wxkq-iE7f-fHcN-Plvm-x61S-iMpJdx
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                256.00 MB
  Current LE             64
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:0
   
  --- Logical volume ---
  LV Name                /dev/mainvg/swap
  VG Name                mainvg
  LV UUID                mZ5mzf-UIj1-p9Yb-8YYJ-jWNZ-pSDW-B11NJc
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                512.00 MB
  Current LE             128
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:1

OLD RESULTS:

root:~# pvdisplay 
  --- Physical volume ---
  PV Name               /dev/hda2
  VG Name               mainvg
  PV Size               18.61 GB / not usable 0   
  Allocatable           yes 
  PE Size (KByte)       4096
  Total PE              4765
  Free PE               4701
  Allocated PE          64
  PV UUID               YnBe3J-sGJS-SItP-U6sL-A0C7-ILMJ-4SdvWI
   
root:~# vgdisplay
  --- Volume group ---
  VG Name               mainvg
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  2
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                1
  Open LV               0
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               18.61 GB
  PE Size               4.00 MB
  Total PE              4765
  Alloc PE / Size       64 / 256.00 MB
  Free  PE / Size       4701 / 18.36 GB
  VG UUID               coGInd-nx6U-0gnq-o5rS-LV9X-6uHp-P8mbOM
   
root:~# lvdisplay
  --- Logical volume ---
  LV Name                /dev/mainvg/root
  VG Name                mainvg
  LV UUID                TsqTw4-Wxkq-iE7f-fHcN-Plvm-x61S-iMpJdx
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                256.00 MB
  Current LE             64
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:0