Network block device

Hexdump of the Initialization / Handshake between a Network Block Device client and server

On Linux, network block device (NBD) is a network protocol that can be used to forward a block device (typically a hard disk or partition) from one machine to a second machine. As an example, a local machine can access a hard disk drive that is attached to another computer.

The protocol was originally developed for Linux 2.1.55 and released in 1997.^[1] In 2011 the protocol was revised, formally documented, and is now developed as a collaborative open standard. There are several interoperable clients and servers.

There are Linux-compatible NBD implementations for FreeBSD and other operating systems. The term 'network block device' is sometimes also used generically.

Technically, a network block device is realized by three components: the server part, the client part, and the network between them. On the client machine, on which is the device node, a kernel driver controls the device. Whenever a program tries to access the device, the kernel driver forwards the request (if the client part is not fully implemented in the kernel it can be done with help of a userspace program) to the server machine, on which the data resides physically. On the server machine, requests from the client are handled by a userspace program.

Network block device servers are typically implemented as a userspace program running on a general-purpose computer. All of the function specific to network block device servers can reside in a userspace process because the process communicates with the client via conventional sockets and accesses the storage via a conventional file system interface.

The network block device client module is available on some Unix-like operating systems, including Linux and Bitrig.^[2] Since the server is a userspace program, it can potentially run on every Unix-like platform; for example, NBD's server part has been ported to Solaris.^[3]

Alternative protocols[]

• iSCSI: The "target-utils" iscsi package on many Linux distributions.
• NVMe-oF: an equivalent mechanism, exposing block devices as NVMe namespaces over TCP, Fibre Channel, RDMA, &c., native to most operating systems
• Loop device: a similar mechanism, but uses a local file instead of a remote one
• DRBD: Distributed Replicated Block Device is a distributed storage system for the Linux platform
• ATA over Ethernet: send ATA commands over Ethernet
• : A protocol that provides network access to USB devices via IP.^[4][5]

External links[]

• Linux Network Block Device on SourceForge: protocol standard and canonical client and server
• nbdkit is a plugin-based NBD server and libnbd is a high-performance C client
• NBD client for FreeBSD on GitHub using GEOM
• qemu-nbd A nbd tool from qemu project
• xNBD is another NBD server program for Linux
• BNBD is an alternative NBD server implementation
• The Network Block Device, the Linux Journal

References[]