FreeBSD/Linux Kernel Cross Reference
sys/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/.

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