Loop Device - Availability

Availability

Some confusion exists about the naming of the loop device under various operating systems. Various Unix-like operating systems provide the loop device functionality under different names.

In Linux, device names are encoded in the symbol table entries of their corresponding device drivers. The device is called "loop" device and device nodes are usually named /dev/loop0, /dev/loop1, etc. They can be created by the makedev script for the static device directory, dynamically by the facilities of the device filesystem (udev), or directly by the mknod command. The management user interface for the loop device is losetup and is part of the util-linux package.

Sometimes, the loop device is erroneously referred to as 'loopback' device, but this term is reserved for a networking device in the Linux kernel (cf. loopback). The concept of the 'loop' device is distinct from that of 'loopback', although similar in name.

In BSD-derived systems, such as NetBSD and OpenBSD, the loop device is called "virtual node device" or "vnd", and generally located at /dev/vnd0, /dev/rvnd0 or /dev/svnd0, etc., in the file system. The vnconfig program is used for configuration.

FreeBSD followed the same conventions as other BSD systems until release version 5, in which the loop device was incorporated into the memory disk driver ("md"). Configuration is now performed using the mdconfig program.

In Solaris/OpenSolaris, the loop device is called "loopback file interface" or lofi, and located at /dev/lofi/1, etc. SunOS has the lofiadm configuration program. The "lofi" driver supports read-only compression and read-write encryption. There is also a 3rd party "fbk" (File emulates Blockdevice) driver available for SunOS/Solaris since summer 1988.

Mac OS X implements a native image mounting mechanism as part of its random access disk device abstraction. The devices appear in /dev as regular disk devices; reads from and writes to those devices are sent to a user-mode helper process, which reads the data from the file or writes it to the file. In the user interface it is automatically activated by opening the disk image. OS X can handle disk (.dmg or .iso), CD-ROM or DVD images in various formats.

Loop mounting is not natively available on Microsoft Windows operating systems (until version Windows 7, where this functionality is natively implemented, and available through the diskpart utility). However, the facility is often added using third-party applications such as Daemon Tools and Alcohol 120%. Freely-available tools from VMware (Disk Mount Utility) and LTR Data (ImDisk) can also be used to achieve similar functionality.

In A2 BlueBottle loop mounting is available via the creation/installation of a so called virtual disk by one of the commands VirtualDisks.Create or VirtualDisks.Install.