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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 think compiled in kernel, linux can use remote
server as one of its block devices. So every time client computer
wants to read /dev/nd0, it will send request over TCP to server, which
will reply with data readed. This can be used for stations with
low-disk space (or even disklesses - if you boot from floppy) to
borrow disk space from other computer. Unlike NFS, it is possible to
put any filesystem on it etc. It is impossible to use NBD as root
filesystem, since it requires user-level program to start. It also
allows you to run block-device in user land (making server and client
physicaly same computer, communicating using loopback).
Current state: It currently works. Network block device looks like
being pretty stable. I originaly 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 actuall kernel driver. This way, kernel does not have to
care about connecting etc. Protocol is rather simple: If 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 copyied 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://atrey.karlin.mff.cuni.cz/~pavel.
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