nbd-client - connect to a server running nbd-server(1), to use its
exported block device
nbd-client [ bs=blocksize ] [ timeout=seconds ] host port nbd-device [
-sdp ] [ -swap ] [ -persist ] [ -nofork ]
nbd-client -d nbd-device
nbd-client -c nbd-device
With nbd-client, you can connect to a server running nbd-server, thus
using raw diskspace from that server as a blockdevice on the local
To do this, support from the Linux Kernel is necessary, in the form of
the Network Block Device (NBD). When you have that, either in the
kernel, or as a module, you can connect to an NBD server and use its
exported file through a block special file with major mode 43.
The following options are supported:
Use a blocksize of "blocksize". Default is 1024; allowed values
are either 512, 1024, 2048 or 4096
host The hostname of the machine running nbd-server.
Set the connection timeout to "seconds". For this to work, you
need a kernel with support for the NBD_SET_TIMEOUT ioctl; this
was introduced into Linus’ tree on 2007-10-11, and will be part
of kernel 2.6.24.
port The TCP port on which nbd-server is running at the server.
The block special file this nbd-client should connect to.
-c Check whether the specified nbd device is connected.
If the device is connected, nbd-client will exit with an exit
state of 0 and print the PID of the nbd-client instance that
connected it to stdout.
If the device is not connected or does not exist (for example
because the nbd module was not loaded), nbd-client will exit
with an exit state of 1 and not print anything on stdout.
If an error occurred, nbd-client will exit with an exit state of
2, and not print anything on stdout either.
-d Disconnect the specified nbd device from the server
When this option is specified, nbd-client will immediately try
to reconnect an nbd device if the connection ever drops
unexpectedly due to a lost server or something similar.
-sdp Connect to the server using the Socket Direct Protocol (SDP),
rather than IP. See nbd-server(1) for details.
-swap Specifies that this NBD device will be used as swapspace. This
option attempts to prevent deadlocks by performing mlockall() at
an appropriate time. It does not however guarantee that such
deadlocks can be avoided.
Specifies that the NBD client should not detach and daemonize
itself. This is mostly useful for debugging.
Some examples of nbd-client usage:
· To connect to a server running on port 2000 at host
"server.domain.com", using the client’s block special file
nbd-client server.domain.com 2000 /dev/nb0
· To connect to a server running on port 2001 at host
"swapserver.domain.com", using the client’s block special file
"/dev/nb1", for swap purposes:
nbd-client swapserver.domain.com 2001 /dev/nb1 -swap
· To disconnect the above connection again (after making sure the block
special file is not in use anymore):
nbd-client -d /dev/nb1
The NBD kernel module and the NBD tools have been written by Pavel
The kernel module is now maintained by Paul Clements
(Paul.Clements@steeleye.com), while the userland tools are maintained
by Wouter Verhelst (email@example.com)
This manual page was written by Wouter Verhelst (<firstname.lastname@example.org>)
for the Debian GNU/Linux system (but may be used by others).
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU General Public License, version 2, as
published by the Free Software Foundation.
03 September 2009