Root/Documentation/blockdev/nbd.txt

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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