1 Network Block Device (TCP version)
2
3 Note: Network Block Device is now experimental, which approximately
4 means, that it works on my computer, and it worked on one of school
5 computers.
6
7 What is it: With this compiled in the kernel, Linux can use a remote
8 server as one of its block devices. So every time the client computer
9 wants to read /dev/nd0, it sends a request over TCP to the server, which
10 will reply with the data read. This can be used for stations with
11 low disk space (or even diskless - if you boot from floppy) to
12 borrow disk space from another computer. Unlike NFS, it is possible to
13 put any filesystem on it etc. It is impossible to use NBD as a root
14 filesystem, since it requires a user-level program to start. It also
15 allows you to run block-device in user land (making server and client
16 physically the same computer, communicating using loopback).
17
18 Current state: It currently works. Network block device looks like
19 being pretty stable. I originally thought that it is impossible to swap
20 over TCP. It turned out not to be true - swapping over TCP now works
21 and seems to be deadlock-free, but it requires heavy patches into
22 Linux's network layer.
23
24 Devices: Network block device uses major 43, minors 0..n (where n is
25 configurable in nbd.h). Create these files by mknod when needed. After
26 that, your ls -l /dev/ should look like:
27
28 brw-rw-rw- 1 root root 43, 0 Apr 11 00:28 nd0
29 brw-rw-rw- 1 root root 43, 1 Apr 11 00:28 nd1
30 ...
31
32 Protocol: Userland program passes file handle with connected TCP
33 socket to actual kernel driver. This way, the kernel does not have to
34 care about connecting etc. Protocol is rather simple: If the driver is
35 asked to read from block device, it sends packet of following form
36 "request" (all data are in network byte order):
37
38 __u32 magic; must be equal to 0x12560953
39 __u32 from; position in bytes to read from / write at
40 __u32 len; number of bytes to be read / written
41 __u64 handle; handle of operation
42 __u32 type; 0 = read
43 1 = write
44 ... in case of write operation, this is
45 immediately followed len bytes of data
46
47 When operation is completed, server responds with packet of following
48 structure "reply":
49
50 __u32 magic; must be equal to
51 __u64 handle; handle copied from request
52 __u32 error; 0 = operation completed successfully,
53 else error code
54 ... in case of read operation with no error,
55 this is immediately followed len bytes of data
56
57 For more information, look at http://atrey.karlin.mff.cuni.cz/~pavel.
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