Device mapper.md


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---
tags:
  - linux
  - devices
  - kernel
---
Device mapper is a kernel driver to map physical block devices to virtual ones; this means that for example we can have many physical devices be represented by a single virtual one (linear mapping).

How does it work?
What is its function in relation to multipath?

Logical devices using device-mapper can be managed using `man 8 dmsetup`
```
[jasras@n04.compute.vps2-lej1 ~]$ sudo dmsetup info 3600a098038314d736724566a67346538
Name:              3600a098038314d736724566a67346538
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        1
Event number:      3
Major, minor:      253, 12
Number of targets: 1
UUID: mpath-3600a098038314d736724566a67346538
```

If dmsetup cannot remove a device because a process still has it open, but lsof does not show any processes that open it, use `-f` which replaces the device with a fake that rejects all I/O.

---
[device-mapper](https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/7/html/logical_volume_manager_administration/device_mapper#device_mapper)
[dmsetup](https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/7/html/logical_volume_manager_administration/dmsetup#dmsetup)