Image backups with netcat

Purpose

This is very useful when working with Virtual Machines or block devices in general. You often need to either transfer the contents of a large block device between machines, back up a block device, or make a copy of a block device.

Prerequisities

Two Linux systems with "netcat" installed.

Step by step

Let's assume you want to make a copy of an LVM partition on a different machine.

• Take a look at the source server and identify the partition you're looking to copy.
  

  root@noded:~# lvs
    LV          VG   Attr   LSize   Origin Snap%  Move Log Copy% 
    hydra       vols -wi-ao  10.00G                              
    mgm2        vols -wi-ao   4.00G                              
    relay       vols -wi-ao   5.00G                                
    swap        vols -wi-a- 512.00M                              
    zeo2        vols -wi-ao  40.00G                          

• Take "relay" in this instance, we will need a 5G partition on the target server. On the target server, create a new LVM partition with the following command;
  

  lvcreate -L 5G -n relay_copy vols
  # in this instance;
  # 5G is the desired size (make sure it's large enough!)
  # relay_copy is the name of the target block device
  # vols is a volume group on the target system with at least 5G of free space ..
  

• Now set up a "listener" on the target system which will pipe it's output into your new block device.
  

  netcat -l -p 8009 > /dev/vols/relay_copy
  # -l = listen
  # -p 8009 = on port 8009
  # > /dev/vols/relay_copy = pipe anything that comes in to our new block device
  

• Now back on the source server, you can use netcat to pipe the contents of the block device out over the network
  

  cat /dev/vols/relay | netcat <dest ip> 8009
  # copy the contents of /dev/vols/relay to <dest ip> on port 8009
  

• Note that the netcat "may" not exit when it's finished, so you can use "vmstat" on the target machine to monitor disk traffic, when the traffic stops simply use "Ctrl-C" on the source system to terminate the process.
  [Anyone know a sure-fire syntax that will make the "cat" command on the source system exit properly?]

Further information

See "man netcat" for more information.