Ported from DragonFlyBSD, but this target had originally existed in
Linux kernel. See below for details.
https://www.kernel.org/doc/Documentation/device-mapper/dm-flakey.txt
Due to two technical issues, this hasn't been hooked to dm.kmod yet.
1) "tick" in hz(9) not working.
2) Unable to use ->b_private in nestiobuf callback when it's already
used for mbp (see HAS_BUF_PRIV2).
taken-from: DragonFlyBSD