Network block device (NBD) is a remote data storage technique used on Linux and Unix systems, allowing a client to access a data store on a remote system as if it were a local disk drive. NBD is similar to NFS but uses TCP for more accurate data transfer. NBD can also be used for RAID configurations, such as DRBD, for high availability.
A network block device (NBD) is a remote data storage technique used on Linux® and Unix® computer systems, which allows a client computer to access a data store on a remote system. Once the client computer has configured the NBD, it is used as if it were actually a disk drive on the client rather than somewhere else on the network. The network block device on the server can be a real hard drive or even a special type of file that can be accessed as if it were a disk. While slower than a local hard drive, the method is useful for a variety of cases, such as backup or compact computers without a hard drive.
Network block device usage, in most cases, involves a server and client model. The so-called device node is maintained on the server. This is typically a hard drive, disk array, or file type often referred to as a disk image. The server runs a small piece of software, called a daemon, which allows a client to access the device node from the server and mount it locally. From the perspective of the client computer, the device node is accessed as if it were any other disk drive.
The network block device is similar, in many ways, to another technique used by Unix® systems known as the network file system (NFS). One main difference, however, is the protocol over which the communication takes place. NFS uses User Datagram Protocol (UDP), while NBD uses Transmission Control Protocol (TCP). Since UDP is a “fire and forget” method of transferring packets over the network, it is sometimes hindered by the client having to ask for data to be retransmitted. TCP communication, on the other hand, establishes a dedicated connection between client and server, ensuring that NBD data is read and written accurately.
This capability in a network block device implementation allows you to establish some special types of disk mirroring between the client and the server. This technique is known as redundant array of independent disks (RAID). With a RAID type one configuration, the data on one disk is mirrored onto any number of additional disks, providing an instantly accessible backup should any of the disks fail. In essence, the array is seen by the computer’s operating system as a single disk.
One such product that makes extensive use of the network block device technique is known as a distributed replicated block device (DRBD®). A DRBD® configuration is often used for very large disk arrays that require high availability. In this case, however, a disk array is set up in a RAID format and then mirrored to other disk arrays through the use of an NDB. The DRBD® is then accessible to any number of client computers.
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