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|0.2.1||Nov 10, 2015|
#15 in Unix APIs
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Low-level devicemapper configuration of the running kernel.
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)
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
Devices have "active" and "inactive" mapping tables. See function descriptions for which table they affect.
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
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::list_devices()and track the
event_nrs for any DM devices of interest.
DM's file descriptor, obtained by calling
- 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 compare
event_nrreturned by the more recent call with
event_nrvalues from the earlier call. If
event_nrdiffers, an event has occurred on that specific device. Handle the event(s). Update the list of last-seen
- Optionally loop and re-invoke
poll()on the fd to wait for more events.