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nexedi
linux
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f4c39f4b
Commit
f4c39f4b
authored
Jul 10, 2003
by
Andrew Morton
Committed by
Linus Torvalds
Jul 10, 2003
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[PATCH] NBD documentation update
From: Paul Clements <Paul.Clements@SteelEye.com> Modernise nbd.txt a bit.
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Documentation/nbd.txt
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f4c39f4b
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.
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