Crate devicemapper[−][src]
Expand description
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.
Polling for Events
Since DM minor version 37, first available in Linux kernel 4.14, the file
descriptor associated with a DM
context may be polled for events generated by
DM devices.
The fd will indicate POLLIN if any events have occurred on any DM devices
since the fd was opened, or since DM::arm_poll()
was called. Therefore,
in order to determine which DM devices have generated an event, the
following usage is required:
- Create a
DM
. - Call
DM::list_devices()
and track theevent_nr
s for any DM devices of interest. poll()
on theDM
’s file descriptor, obtained by callingDM::file().as_raw_fd()
.- If the fd indicates activity, first clear the event by calling
DM::arm_poll()
. This must be done before event processing to ensure events are not missed. - Process events. Call
DM::list_devices()
again, and compareevent_nr
returned by the more recent call withevent_nr
values from the earlier call. Ifevent_nr
differs, an event has occurred on that specific device. Handle the event(s). Update the list of last-seenevent_nr
s. - Optionally loop and re-invoke
poll()
on the fd to wait for more events.
Modules
International Electrotechnical Commission Units Standards
Structs
A type for bytes
DM Cache device
Cache dev performance data
Cache usage
Status values of a cache device when it is working
Context needed for communicating with devicemapper.
A type for data blocks
A struct containing the device’s major and minor numbers
Flags used by devicemapper, see: https://sourceware.org/git/?p=lvm2.git;a=blob;f=libdm/libdevmapper.h#l3627 for complete information about the meaning of the flags.
Flags used by devicemapper.
The borrowed version of the DM identifier.
The owned version of the DM identifier.
Encapsulates options for device mapper calls
The borrowed version of the DM identifier.
The owned version of the DM identifier.
Target params for flakey target
A DM construct of combined Segments
A target table for a linear device. Such a table allows flakey targets as well as linear targets.
Struct representing params for a linear target
A type for meta blocks
A type for sectors
One line of a device mapper table.
The borrowed version of the DM identifier.
The owned version of the DM identifier.
DM construct for a thin block device
A thindev id is a 24 bit number, i.e., its bit width is not a power of 2.
Status values for a thin device that is working
DM construct to contain thin provisioned devices
Contains values indicating the thinpool’s used vs total allocations for metadata and data blocks.
Status of a working thin pool, i.e, one that does not have status Fail
Enums
Return type of CacheDev::status()
Used as a parameter for functions that take either a Device name or a Device UUID.
Super error type, with constructors distinguishing outer errors from core errors.
A very simple breakdown of outer layer errors.
Target params for linear dev. These are either flakey or linear.
Policy if no space on device
Top-level thinpool status that indicates if it is working or failed.
Indicates if a working thinpool is working optimally, or is experiencing a non-fatal error condition.
Thin device status.
Constants
The maximum size recommended in the docs for a cache block.
The minimum size recommended in the docs for a cache block.
disk sector size in bytes
Traits
A trait capturing some shared properties of DM devices.
Functions
Check if a device of the given name exists.
Get a device number from a device node. Return None if the device is not a block device; devicemapper is not interested in other sorts of devices. Return None if the device appears not to exist.
Type Definitions
return result for DM functions