dm-ioctl – Versioning

• The driver has a version number in the format major.minor.patchlevel-interface (date) – the latest is 4.4.0.

• There are 15 ioctl commands. A common request structure is passed to the kernel. It begins with a version number.

• For the command to proceed:

  • The major number passed in must match. We'll change this if we ever decide it's necessary to make a change to the interface that breaks backwards compatibility.
  • The minor number passed in must be less than or equal to the kernel driver's. We increment this when we extend the interface in some way e.g. by adding a new command.

• libdevmapper in userspace is responsible for adapting to the driver version – e.g. on a current kernel it might be able to do something in a single call, but with an older kernel it might take three.