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dm-ioctl (2)
Only root can use ioctl() on /dev/mapper/control.
It may be possible to support ioctl() on the devices themselves in future e.g. to provide a way for root to delegate device management to other users e.g. database administrators using raw devices.
The driver has a version number in the format major.minor.patchlevel – the latest is 4.4.0.
We ignore the 'size' part of the ioctl definition (_IOC_SIZE).
Requests are of variable size – userspace has to provide sufficient space for the answer, otherwise the answer is truncated, DM_BUFFER_FULL_FLAG gets set and userspace should retry with a larger buffer.