XNBD-SERVER(8) System Manager's Manual XNBD-SERVER(8)
NAME
xnbd-server -- serve a file as a block device to other computers running the GNU/Linux(tm) Operating System
SYNOPSIS
xnbd-server [--bgctlprefix PATH] [--cow] [--daemonize] [--help] [--inetd] [--logpath PATH] [--lport PORT] [--proxy] [--readonly]
[--target] [--version] FILE
xnbd-server [--proxy] [options] TARGET_HOST TARGET_PORT CACHE_IMAGE BITMAP_IMAGE
xnbd-server [--target] [options] CACHE_IMAGE
xnbd-server [--help] [--version]
DESCRIPTION
xnbd-server is the server for the Linux Network Block Device (NBD). With xNBD, a client can use a file, exported over the network from a
server, as a block device. It can then be used for whatever purpose a normal block device (hard disk, CD-ROM, ...) can be used for.
xNBD can be useful for diskless clients that need swap space, but you can also create a file system on it and use it as though it were a
local file system.
xNBD offers several advantages over the original NBD implementation. In addition to features offered by NBD, xNBD supports better perfor-
mance, (distributed) Copy-on-Write, (basic) snapshot support and Live Storage Migration along with IPv6 support.
xNBD can operate in two modes, the target mode and the alternative proxy mode. In target mode, xNBD offers a listening TCP connection
which allows a xnbd-client(1) to access the exported CACHE_IMAGE. In proxy mode, xNBD connects another running xnbd-server instance and
acts as transparent proxy to the origin target server. This is very useful for storage migration for Virtual Machine Monitors.
OPTIONS
The following options are supported:
--bgctlprefix PATH
The xNBD server can be controlled through a control socket. When given, use prefix as file name, default is /tmp/xnbd-bg.ctl).
--cow Enable Copy-on-Write mode. When this option is provided, write-operations are not committed to the exported FILE, but to a sepa-
rate file. This separate file is removed when the connection is closed, which means that serving this way will make xnbd-server
slow down (especially on large block devices with lots of writes), and that after disconnecting and reconnecting the client or
the server, all changes are lost.
--daemonize
Go into background as soon as the xnbd-server is started up
--inetd If xNBD shall be run from a Internet super-server, (e.g. inetd), this argument instructs xNBD to redirect output to stderr
instead of a log file.
--logpath FILE
Log informational messages to the given FILE if given. Defaults to /var/log/xnbd-server.log
--lport PORT
Accept incoming connections on the given PORT. A valid port is any number between 1 and 65536. You may not use the same listeing
port more than once per system. If this argument is not given, xNBD will listen on port 8520 by default.
--proxy In the proxy server mode of xNBD, all I/O requests are intercepted, and redirected to a origin target server if needed. All
updated blocks are saved at the proxy server, and read blocks are also cached. No write does not happen at the target server.
This feature is used for distributed Copy-on-Write NBD disks; one read-only disk image is shared among multiple clients, and
updated disk data is saved at each proxy
--readonly
Export the FILE read only. If a client tries to write to a read-only exported file, it will receive an error, but the connection
will stay up.
--target Start an xNBD server in target mode, that is it will export block level access to the given FILE.
WARNING: If you need concurrent access from multiple clients, you can access an exported image concurrently from multiple
clients. Yet xNBD does not do any locking or synchronization. In most cases you WILL need a cluster file system on the exported
FILE image to avoid damage on your data.
POSITIONAL ARGUMENTS
The following positional options are supported:
FILE The filename of the FILE that should be exported. This can be an arbitrary file, including disk images and real block devices
(i.e. a block device like /dev/sda1).
TARGET_HOST
When in proxy mode, use the given TARGET_HOST as origin host. The resulting xnbd-server instance will forward requests transpar-
ently to this remote side.
TARGET_PORT
The target origin port to connect in proxy mode
BITMAP_IMAGE
The bitmap file uses for synchronization when connecting to a target origin host. A bitmap file records block numbers of updated
and cached blocks.
SIGNALS
SIGHUP When in proxy mode, a SIGHUP instructs the nbd-server to detach from its origin. From there and beyond, the nbd server instance
will run as if it would have been started in target mode.
SEE ALSO
xnbd-client (1).
AUTHOR
The NBD kernel module and the NBD tools have been written by Pavel Macheck (pavel@ucw.cz) and is now maintained by Paul Clements.
(Paul.Clements@steeleye.com).
The xNBD userland (client and server) have been written by Takahiro Hirofuchi (t.hirofuchi@aist.go.jp)
This manual page was written by Arno Toell (debian@toell.net) 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 (wouter@debian.org). 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.
XNBD-SERVER(8)