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 Unix-like operating systems, including Linux.