Comment # 1 on bug 1020077 from Stanislav Brabec

fstrim -a has a de-duplication heuristics.

https://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=8dd51c183396c4bf6344d08b01fb84658e0a8eb4

https://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=e05a3400e79e8cba6aca8725fd53d2b9753f0cf3

But adding a printf just before the the FITRIM ioctl() really shows that it
calls FITRIM for each sub-volume.

We should not invent strange workarounds, but fix fstrim -a instead.