[PATCH] NBD documentation update

From: Paul Clements <Paul.Clements@SteelEye.com>

Modernise nbd.txt a bit.

Documentation/nbd.txt

                       Network Block Device (TCP version)
                                        
-   Note: Network Block Device is now experimental, which approximately
-   means, that it works on my computer, and it worked on one of school
-   computers.
-   
-   What is it: With this compiled in the kernel, Linux can use a remote
-   server as one of its block devices. So every time the client computer
-   wants to read /dev/nd0, it sends a request over TCP to the server, which
-   will reply with the data read. This can be used for stations with
-   low disk space (or even diskless - if you boot from floppy) to
-   borrow disk space from another computer. Unlike NFS, it is possible to
-   put any filesystem on it etc. It is impossible to use NBD as a root
-   filesystem, since it requires a user-level program to start. It also
-   allows you to run block-device in user land (making server and client
-   physically the same computer, communicating using loopback).
-   
-   Current state: It currently works. Network block device looks like
-   being pretty stable. I originally thought that it is impossible to swap
-   over TCP. It turned out not to be true - swapping over TCP now works
-   and seems to be deadlock-free, but it requires heavy patches into
-   Linux's network layer.
-   
-   Devices: Network block device uses major 43, minors 0..n (where n is
-   configurable in nbd.h). Create these files by mknod when needed. After
-   that, your ls -l /dev/ should look like:
-
-brw-rw-rw-   1 root     root      43,   0 Apr 11 00:28 nd0
-brw-rw-rw-   1 root     root      43,   1 Apr 11 00:28 nd1
-...
-
-   Protocol: Userland program passes file handle with connected TCP
-   socket to actual kernel driver. This way, the kernel does not have to
-   care about connecting etc. Protocol is rather simple: If the driver is
-   asked to read from block device, it sends packet of following form
-   "request" (all data are in network byte order):
-   
-  __u32 magic;        must be equal to 0x12560953
-  __u32 from;         position in bytes to read from / write at
-  __u32 len;          number of bytes to be read / written
-  __u64 handle;       handle of operation
-  __u32 type;         0 = read
-                      1 = write
-  ...                 in case of write operation, this is
-                      immediately followed len bytes of data
-
-   When operation is completed, server responds with packet of following
-   structure "reply":
-   
-  __u32 magic;        must be equal to
-  __u64 handle;       handle copied from request
-  __u32 error;        0 = operation completed successfully,
-                      else error code
-  ...                 in case of read operation with no error,
-                      this is immediately followed len bytes of data
-
-   For more information, look at http://nbd.sf.net/.
+   What is it: With this compiled in the kernel (or as a module), Linux
+   can use a remote server as one of its block devices. So every time
+   the client computer wants to read, e.g., /dev/nb0, it sends a
+   request over TCP to the server, which will reply with the data read.
+   This can be used for stations with low disk space (or even diskless -
+   if you boot from floppy) to borrow disk space from another computer.
+   Unlike NFS, it is possible to put any filesystem on it, etc. It should
+   even be possible to use NBD as a root filesystem (I've never tried),
+   but it requires a user-level program to be in the initrd to start.
+   It also allows you to run block-device in user land (making server
+   and client physically the same computer, communicating using loopback).
+   
+   Current state: It currently works. Network block device is stable.
+   I originally thought that it was impossible to swap over TCP. It
+   turned out not to be true - swapping over TCP now works and seems
+   to be deadlock-free, but it requires heavy patches into Linux's
+   network layer.
+   
+   For more information, or to download the nbd-client and nbd-server
+   tools, go to http://nbd.sf.net/.
+
+   Howto: To setup nbd, you can simply do the following:
+
+   First, serve a device or file from a remote server:
+
+   nbd-server <port-number> <device-or-file-to-serve-to-client>
+
+   e.g.,
+	root@server1 # nbd-server 1234 /dev/sdb1
+
+	(serves sdb1 partition on TCP port 1234)
+
+   Then, on the local (client) system:
+
+   nbd-client <server-name-or-IP> <server-port-number> /dev/nb[0-n]
+
+   e.g.,
+	root@client1 # nbd-client server1 1234 /dev/nb0
+
+	(creates the nb0 device on client1)
+
+   The nbd kernel module need only be installed on the client
+   system, as the nbd-server is completely in userspace. In fact,
+   the nbd-server has been successfully ported to other operating
+   systems, including Windows.