“‘I’m happy to announce that I’ve implemented a Block I/O
bandwidth controller,’ began Ryo Tsuruta, explaining that it was
intended to be used in a cgroup or virtual machine environment,
implemented as a device-mapper driver. He detailed a token-based
implementation in which dm-band passes out to the various groups,
‘a group passes on I/O requests that its job issues to the
underlying layer so long as it has tokens left, while requests are
blocked if there aren’t any tokens left in the group. One token is
consumed each time the group passes on a request. Dm-band will
refill groups with tokens once all of groups that have requests on
a given physical device use up their tokens…'”