xnbd-client(1) Connect to a server running xnbd-server(8), to use its exported block device


xnbd-client [OPTIONS] [bs=SIZE] [timeout=SECONDS] HOST PORT NBD-DEVICE

xnbd-client [OPTIONS] --connect NBD-DEVICE HOST PORT [HOST PORT ..]

xnbd-client --disconnect NBD-DEVICE

xnbd-client --check NBD-DEVICE

xnbd-client [OPTIONS] --getsize64 HOST PORT [HOST PORT ..]


With xnbd-client, you can connect to a server running xnbd-server or xnbd-wrapper, thus using raw diskspace from that server as a block device on the local client.

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.

Long options can also be specified with two leading dashes. Some options are call-compatible to nbd-client(1) as are most behavioural switches. This makes xnbd-client a plug-in replacement for nbd-client.


The following options are supported:

--blocksize SIZE

Use the provided value as block size. Default is 1024; allowed values are either 512, 1024, 2048 or 4096. For best results use a block size value of 4096.

For compatibility to nbd-client(1) you can also use bs=SIZE.


Check whether the specified nbd device is connected.

If the device is connected, xnbd-client will exit with an exit state of 0 and print the PID of the xnbd-client instance that connected it to stdout.

If the device exists but is not connected (i.e. is free for use), xnbd-client will exit with code 2 and not print anything on stdout.

If the device does not exist (for example because the nbd module was not loaded) or if an error occurred, xnbd-client will exit with a positive return code other than 2 and not print anything on stdout, either.


Connect to the nbd-server


Disconnect the specified nbd device from the server. Terminates execution with an exit state of 0 on success.


Report remote disk size in bytes, mimicking blockdev(8).

--exportname NAME

If the server supports to access devices by an identifier, use NAME to request access to a particular volume. This command is useful in combination with an xnbd-wrapper and only succeeds, if the remote host is exporting the requested device.

--retry COUNT

Try up to "COUNT" times to connect to the associated nbd-server. Default is 1, that is xnbd-client will stop after the first unsuccessful try.

--recovery-command COMMAND

Invoke the specified command on unexpected disconnection

--recovery-command-reboot: Invoke the reboot(8) command on unexpected disconnection

--timeout SECONDS

Use a timeout period (default is 0 which means not to use a timeout). Please do not use this option for now, as it seems not to work due to a bug in the kernel module.

For compatibility to nbd-client(1) you can also use timeout=SECONDS


The following positional options are supported:


The site to connect to a remote xnbd-server. You can specify any resolvable hostname, IPv4 or IPv6 address.


The port number to connect to on remote side


The local nbd-device to be associated with the remote xnbd-server.

You can specify multiple host port tuples. xnbd-client will try to connect to each of them in order until it succeeds to establish a connection to a server.


The NBD device is known to deadlock when not being used altogether with the deadline scheduler. Make sure to do:

echo deadline > /sys/block/nbd0/queue/scheduler


The NBD kernel module and the NBD tools have been written by Pavel Macheck ([email protected]) and is now maintained by Paul Clements ([email protected]).

The xNBD userland programs have been written by Takahiro Hirofuchi ([email protected]).

This manual page was written by Arno Toell ([email protected]) for the Debian GNU/Linux system (but may be used by others). Large parts are verbatim copies of the original nbd-server and nbd-client manual pages written by Wouter Verhelst ([email protected]). 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.