Appendix A. The Device Mapper

Appendix A. The Device Mapper

The Device Mapper is a kernel driver that provides a generic framework for volume management. It provides a generic way of creating mapped devices, which may be used as logical volumes. It does not specifically know about volume groups or metadata formats.

The Device Mapper provides the foundation for a number of higher-level technologies. In addition to LVM, device-mapper multipath and the dmraid command use the Device Mapper.

The user interface to the Device Mapper is the ioctl system call.

LVM logical volumes are activated using the Device Mapper. Each logical volume is translated into a mapped device, Each segment translates into a line in the mapping table that describes the device. The Device Mapper provides linear mapping, striped mapping, and error mapping, amongst others. Two disks can be concatenated into one logical volume with a pair of linear mappings, one for each disk.

The dmsetup command is a command line wrapper for communication with the Device Mapper. It provides complete access to the ioctl commands through the libdevmapper command. For general system information about LVM devices, you may find the dmsetup info command to be useful.

For information about the options and capabilities of the dmsetup command, see the dmsetup(8) man page.

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