Devmapper

Devmapper, short for device-mapper, refers to the Linux kernel Device Mapper framework and its accompanying user-space tools used to manage virtual block devices. The framework provides a generic mechanism to map one or more underlying physical block devices to a single virtual device, enabling features such as logical volumes, snapshots, mirrors, and thin-provisioned pools. It is implemented in the kernel as the device-mapper subsystem and in userland by tools such as dmsetup and the libdevmapper library.

How it works: A user-space process issues commands to the device-mapper control device, typically /dev/mapper/control, to create mappings described by a table. The table specifies a target type (for example linear, snapshot, mirror, or thin-pool) and the underlying devices. The kernel reads the table and attaches a new mapped device at /dev/mapper/<name> that presents a virtual block device to the system.

Usage: The device-mapper framework is the foundation of LVM (logical volume manager) and is used by other storage systems that require flexible mapping. In container environments, Docker historically offered a devmapper storage driver that used a thin-provisioned pool to store images and containers; this approach is less common today, with overlay-based storage drivers being preferred for many setups.

Notes: The project comprises the kernel device-mapper subsystem and userland utilities; performance and reliability depend on the target types used and the underlying hardware.

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