devicemapper 0.4.0

A library for using Linux device mapper

Crate devicemapper [] [src]

Low-level devicemapper configuration of the running kernel.

Overview

Linux's devicemapper allows the creation of block devices whose storage is mapped to other block devices in useful ways, either by changing the location of its data blocks, or performing some operation on the data itself. This is a low-level facility that is used by higher-level volume managers such as LVM2. Uses may include:

  • Dividing a large block device into smaller logical volumes (dm-linear)
  • Combining several separate block devices into a single block device with better performance and/or redundancy (dm-raid)
  • Encrypting a block device (dm-crypt)
  • Performing Copy-on-Write (COW) allocation of a volume's blocks enabling fast volume cloning and snapshots (dm-thin)
  • Configuring a smaller, faster block device to act as a cache for a larger, slower one (dm-cache)
  • Verifying the contents of a read-only volume (dm-verity)

Usage

Before they can be used, DM devices must be created using DM::device_create(), have a mapping table loaded using DM::table_load(), and then activated with DM::device_suspend(). (This function is used for both suspending and activating a device.) Once activated, they can be used as a regular block device, including having other DM devices map to them.

Devices have "active" and "inactive" mapping tables. See function descriptions for which table they affect.

Structs

DM

Context needed for communicating with devicemapper.

Device

A struct containing the device's major and minor numbers

DeviceInfo

Contains information about the device.

DmFlags

Flags used by devicemapper.

Enums

DevId

Used as a parameter for functions that take either a Device name or a Device UUID.

Constants

DM_ACTIVE_PRESENT

Out: Active table is present.

DM_BUFFER_FULL

Out: Passed-in buffer was too small.

DM_DATA_OUT

Out: A message generated output data.

DM_DEFERRED_REMOVE

In: Do not remove in-use devices. Out: Device scheduled to be removed when closed.

DM_INACTIVE_PRESENT

Out: Inactive table is present.

DM_INTERNAL_SUSPEND

Out: Device is suspended internally.

DM_NOFLUSH

In: Suspend without flushing queued I/Os.

DM_PERSISTENT_DEV

In: Use passed-in minor number.

DM_QUERY_INACTIVE_TABLE

In: Query inactive table instead of active.

DM_READONLY

In: Device should be read-only. Out: Device is read-only.

DM_SECURE_DATA

In: All buffers are wiped after use. Use when handling crypto keys.

DM_SKIP_BDGET

Obsolete.

DM_SKIP_LOCKFS

In: Avoid freezing filesystem when suspending.

DM_STATUS_TABLE

In: STATUS command returns table info instead of status.

DM_SUSPEND

In: Device should be suspended. Out: Device is suspended.

DM_UEVENT_GENERATED

Out: A uevent was generated, the caller may need to wait for it.

DM_UUID

In: Rename affects UUID field, not name field.

Functions

dev_majors

Major numbers used by DM.

Type Definitions

TargetLine

This 4-tuple consists of starting offset (sectors), length (sectors), target type (string, e.g. "linear"), and params(string). See target documentation for the format of each target type's params field.