Device-mapper targets

The device-mapper is a framework of the Linux kernel that allows the mapping of physical block devices up to higher-level virtual block devices. It forms the basis for the logical volume manager (LVM), Linux Software RAIDs and dm-crypt encryption and offers additional functions such as file system snapshots. This wiki article gives an overview of the device mapper targets.

BIO-based device-mapper targets

Targets for production use

• dm-cache - aims to improve the performance of a block device (e.g. a spindle) by dynamically shifting part of the data to a faster, smaller device (e.g. an SSD).
• dm-clone - creates a one-to-one copy of an existing, write-protected source device into a writable target device.
• dm-crypt - offers transparent encryption of block devices using the Kernel Crypto API.
• dm-ebs - emulates a smaller logical block size of a device with a larger logical block size, the main purpose is the emulation of 512-byte sectors of native 4K hard disks.
• dm-era - behaves similarly to dm-linear, recording information on changed blocks (used for backup software, for example).
• dm-integrity - emulates a block device that has additional sector-related tags that can be used to store integrity information.
• dm-linear - maps a linear area of the device mapper device to a linear area of another device (basic functionality for LVM).
• dm-mirror- has a mechanism for copying/mirroring in the background, writes all mirrors.
• dm-raid - Interface to the software raid driver of the kernel.
• dm-stripe - provides a striped device (e.g. RAID 0) via one or more underlying devices.
• dm-switch - creates a device that supports arbitrary mapping of fixed-size I/O areas to a fixed set of paths (e.g. for Dell EqualLogic and some other iSCSI storage arrays).
• dm-thin - Target for thin provisioning.
• dm-unstripe - provides a transparent mechanism to split a striped device (dm-striped target or a hardware RAID-0) into the individual stripes (to access the underlying hard disks without having to touch the actual backing block device).
• dm-vdo - (virtual data optimizer) offers deduplication, compression and thin provisioning at block level.
• dm-verity - provides a transparent integrity check of block devices using a cryptographic digest provided by the Kernel Crypto API (this target is read-only).
• dm-writecache - writes to peristent memory or SSD.
• dm-zoned - represents a zoned block device (ZBC- and ZAC-compliant devices) as a regular block device without write pattern restrictions.

Targets for debugging use

• dm-delay - delays read and/or write operations and assigns them to different devices.
• dm-dust - emulates the behavior of faulty sectors, behaves similarly to dm-linear.
• dm-flaky - periodically emulates an unreliable behavior to simulate failing devices for test purposes, behaves similar to dm-linear.
• dm-zero - always returns NULL data during read operations and ignores write accesses, behaves similarly to /dev/zero, but works as a block device (instead of a character device like /dev/zero).

Request-based device-mapper targets

• dm-multipath - Host-side logic for using multiple (redundant) paths to a storage device.

Further information

• Device-mapper (wiki.gentoo.org)

References

Author: Werner Fischer Werner Fischer, working in the Knowledge Transfer team at Thomas-Krenn, completed his studies of Computer and Media Security at FH Hagenberg in Austria. He is a regular speaker at many conferences like LinuxTag, OSMC, OSDC, LinuxCon, and author for various IT magazines. In his spare time he enjoys playing the piano and training for a good result at the annual Linz marathon relay.

Translator: Alina Ranzinger Alina has been working at Thomas-Krenn.AG since 2024. After her training as multilingual business assistant, she got her job as assistant of the Product Management and is responsible for the translation of texts and for the organisation of the department.