linux/Documentation/blockdev/nbd.txt
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   1                      Network Block Device (TCP version)
   2                                       
   3   What is it: With this compiled in the kernel (or as a module), Linux
   4   can use a remote server as one of its block devices. So every time
   5   the client computer wants to read, e.g., /dev/nb0, it sends a
   6   request over TCP to the server, which will reply with the data read.
   7   This can be used for stations with low disk space (or even diskless -
   8   if you boot from floppy) to borrow disk space from another computer.
   9   Unlike NFS, it is possible to put any filesystem on it, etc. It should
  10   even be possible to use NBD as a root filesystem (I've never tried),
  11   but it requires a user-level program to be in the initrd to start.
  12   It also allows you to run block-device in user land (making server
  13   and client physically the same computer, communicating using loopback).
  14   
  15   Current state: It currently works. Network block device is stable.
  16   I originally thought that it was impossible to swap over TCP. It
  17   turned out not to be true - swapping over TCP now works and seems
  18   to be deadlock-free, but it requires heavy patches into Linux's
  19   network layer.
  20   
  21   For more information, or to download the nbd-client and nbd-server
  22   tools, go to http://nbd.sf.net/.
  23
  24   Howto: To setup nbd, you can simply do the following:
  25
  26   First, serve a device or file from a remote server:
  27
  28   nbd-server <port-number> <device-or-file-to-serve-to-client>
  29
  30   e.g.,
  31        root@server1 # nbd-server 1234 /dev/sdb1
  32
  33        (serves sdb1 partition on TCP port 1234)
  34
  35   Then, on the local (client) system:
  36
  37   nbd-client <server-name-or-IP> <server-port-number> /dev/nb[0-n]
  38
  39   e.g.,
  40        root@client1 # nbd-client server1 1234 /dev/nb0
  41
  42        (creates the nb0 device on client1)
  43
  44   The nbd kernel module need only be installed on the client
  45   system, as the nbd-server is completely in userspace. In fact,
  46   the nbd-server has been successfully ported to other operating
  47   systems, including Windows.
  48
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