Английская Википедия:Device mapper
Шаблон:Short description The device mapper is a framework provided by the Linux kernel for mapping physical block devices onto higher-level virtual block devices. It forms the foundation of the logical volume manager (LVM), software RAIDs and dm-crypt disk encryption, and offers additional features such as file system snapshots.[1]
Device mapper works by passing data from a virtual block device, which is provided by the device mapper itself, to another block device. Data can be also modified in transition, which is performed, for example, in the case of device mapper providing disk encryption or simulation of unreliable hardware behavior.
This article focuses on the device mapper implementation in the Linux kernel, but the device mapper functionality is also available in both NetBSD and DragonFly BSD.[2][3]
Usage
Applications (like LVM2 and Enterprise Volume Management System (EVMS)) that need to create new mapped devices talk to the device mapper via the libdevmapper.so shared library, which in turn issues ioctls to the /dev/mapper/control device node.[4] Configuration of the device mapper can be also examined and configured interactivelyШаблон:Mdashbor from shell scriptsШаблон:Mdashbby using the Шаблон:Man utility.[5][6]
Both of these two userspace components have their source code maintained alongside the LVM2 source.[7]
Features
Functions provided by the device mapper include linear, striped and error mappings, as well as crypt and multipath targets. For example, two disks may be concatenated into one logical volume with a pair of linear mappings, one for each disk. As another example, crypt target encrypts the data passing through the specified device, by using the Linux kernel's Crypto API.[1]
Шаблон:As of, the following mapping targets are available:[1][5]
- cacheШаблон:Snd allows creation of hybrid volumes, by using solid-state drives (SSDs) as caches for hard disk drives (HDDs)
- cloneШаблон:Snd will permit usage before a transfer is complete.
- cryptШаблон:Snd provides data encryption, by using the Linux kernel's Crypto API
- delayШаблон:Snd delays reads and/or writes to different devices (used for testing)
- eraШаблон:Snd behaves in a way similar to the linear target, while it keeps track of blocks that were written to within a user-defined period of time[9]
- errorШаблон:Snd simulates I/O errors for all mapped blocks (used for testing)
- flakeyШаблон:Snd simulates periodic unreliable behaviour (used for testing)
- linearШаблон:Snd maps a continuous range of blocks onto another block device
- mirrorШаблон:Snd maps a mirrored logical device, while providing data redundancy
- multipathШаблон:Snd supports the mapping of multipathed devices, through usage of their path groups
- raidШаблон:Snd offers an interface to the Linux kernel's software RAID driver (md)
- snapshot and snapshot-originШаблон:Snd used for creation of LVM snapshots, as part of the underlying copy-on-write scheme
- stripedШаблон:Snd stripes the data across physical devices, with the number of stripes and the striping chunk size as parameters
- thin Шаблон:Snd allows creation of devices larger than the underlying physical device, physical space is allocated only when written to
- zeroШаблон:Snd an equivalent of
/dev/zero, all reads return blocks of zeros, and writes are discarded
Шаблон:AnchorApplications
Linux kernel features and projects built on top of the device mapper include the following:
- cryptsetupШаблон:Snd utility used to conveniently setup disk encryption based on dm-crypt
- dm-crypt/LUKSШаблон:Snd mapping target that provides volume encryption
- dm-cacheШаблон:Snd mapping target that allows creation of hybrid volumes
- dm-integrityШаблон:Snd mapping target that provides data integrity, either using checksumming or cryptographic verification,[10][11] also used with LUKS[12]
- dm-log-writesШаблон:Snd mapping target that uses two devices, passing through the first device and logging the write operations performed to it on the second device[13]
- dm-verityШаблон:Snd validates the data blocks contained in a file system against a list of cryptographic hash values, developed as part of the ChromiumOS project[14]
- Шаблон:ManШаблон:Snd provides access to "fake" RAID configurations via the device mapper
- DM MultipathШаблон:Snd provides I/O failover and load-balancing of block devices within the Linux kernel
- DockerШаблон:Snd uses device mapper to create copy-on-write storage for software containers
- DRBD (Distributed Replicated Block Device)
- EVMS (deprecated)
- Шаблон:ManШаблон:Snd utility called from hotplug upon device maps creation and deletion
- LVM2Шаблон:Snd logical volume manager for the Linux kernel
- VeraCrypt - Linux version of TrueCrypt
- VDO - Virtual Data Optimizer
References
External links
- Device mapper home at Red Hat Шаблон:Webarchive
- Шаблон:Cite webШаблон:Snd an article illustrating the use of various device mapper targets
- userspace tool to set up software RAID using various RAID metadata formats
- Multipath support in the device mapper, LWN.net, February 23, 2005, by Jonathan Corbet
Шаблон:Linux kernel Шаблон:Linux Шаблон:Operating system
