Ubuntu Manpage: xnbd-server - Serve a file as a block device to other computers via the NBD protocol

NAME

       xnbd-server - Serve a file as a block device to other computers via the NBD protocol

SYNOPSIS

       xnbd-server --target [options] DISK_IMAGE

       xnbd-server --cow-target [options] BASE_DISK_IMAGE

       xnbd-server --proxy [options] [--target-exportname NAME] REMOTE_HOST REMOTE_PORT CACHE_IMAGE
       CACHE_BITMAP_IMAGE CONTROL_SOCKET_PATH

       xnbd-server --help

       xnbd-server --version

DESCRIPTION

xnbd-server is a server program of the Network Block Device (NBD) protocol. It exports an image file to
the clients of the NBD protocol over network. A client node can access to the exported file through a
block-level I/O interface; it is possible to create swap areas on it or create file systems on it.

The xNBD (i.e. extended NBD) programs offer several advantages over the original NBD implementation. In
addition to the original features, xNBD is intended to achieve better performance. It supports
(distributed) copy-on-write, (basic) snapshot support, live storage migration for virtual machines, and
IPv6 networking.

xnbd-server can operate in three modes (i.e., the target mode, the copy-on-write target mode, and the
proxy mode).

xnbd-server --target exports DISK_IMAGE to clients.

xnbd-server --cow-target exports BASE_DISK_IMAGE to clients. Write operations are not committed to the
exported disk image, but to a separate file. This separate file is removed when the server instance is
shutdown, and all changes are lost.

xnbd-server --proxy provides the image server of the NBD protocol, but it actually works as a proxy to a
remote xnbd-server specified with REMOTE_HOST and REMOTE_PORT.

The proxy server receives read/write requests from clients as a normal image server does. But, it locally
caches disk blocks, and retrieves disk blocks from the remote server if necessary. No write operation
does not happen at the remote server.

Cached blocks are saved in CACHE_DISK_IMAGE. The block numbers of cached blocks are saved in
CACHE_BITMAP_IMAGE. The proxy server is controlled by xnbd-bgctl(1) through CONTROL_SOCKET_PATH.

The proxy server can be used to speed up remote access, share a read-only disk image among multiple
servers and clients, and replicate an exported image to another node transparently. It also works for
live storage migration of virtual machines.

Warning
Multiple clients can concurrently access to a single server instance. Yet xnbd-server does not offer
any locking or synchronization mechanism among concurrent clients. In most cases you WILL need a
cluster file system on the exported image to avoid damage to your data.

OPTIONS

       The following options are supported:

       --daemonize
           Run xnbd-server in the background

       --inetd
           Run xnbd-server for an Internet super-server daemon (e.g., inetd). All network I/O is redirected to
           the stdin descriptor.

       --logpath FILE
           Log informational messages to file FILE rather than stderr/syslog.

       --syslog
           Log informational messages to syslog.

       --lport PORT
           Listen incoming connections on the given PORT. If this argument is not given, xnbd-server will listen
           on port 8520 by default.

       --readonly
           Export the image file as read-only. If receiving a write request from a client, the server
           disconnects the client immediately. If this option is used in the proxy mode, xnbd-server rejects
           write requests from clients, but accepts read requests and may cache new blocks. The cache image file
           is updated if new blocks are cached.

       --connected-fd NUMBER
           Use file descriptor NUMBER as a bi-directional, pre-negotiated channel to a single client. Used by
           xnbd-wrapper(8) on invocation of xnbd-server, internally. The file descriptor is turned to blocking
           mode by xnbd-server before usage.

OPTIONS (PROXY MODE ONLY)

       --target-exportname NAME
           Set the export name to request from an xnbd-wrapper target.

       --clear-bitmap
           Clear an existing bitmap file. By default, previous state is re-used.

       --max-queue-size NUMBER
           Limit the total number of queued requests to NUMBER. If the current number of queued requests reaches
           this limitation, the server delays receiving new requests. By default (i.e., 0), there is no
           limitation. Use this option to keep memory usage in a safe level if a client asynchronously sends a
           large number of requests.

       --max-buf-size NUMBER
           Limit the usage of internal buffer to approximately NUMBER bytes. If the current buffer usage reaches
           this limitation, the server delays receiving new requests. By default (i.e., 0), there is no
           limitation. Use this option to keep memory usage in a safe level if a client asynchronously sends a
           large number of requests.

SIGNALS

       SIGUSR1
           Take a snapshot of the image file. Currently, this feature works only in the target mode.

       SIGUSR2
           Change the proxy mode to the target mode. Use xnbd-bgctl --switch instead of sending this signal.

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 programs have been written by Takahiro Hirofuchi (t.hirofuchi@aist.go.jp).

       This manual page was written by Arno Toell (arno@debian.org) 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.