LVM mirroring across VGs (enables eg. online FS migration from one vg to another)

LVM is great and has been for decades. Many LVM manipulations can be done online without unmounting filesystems or stopping the applications, which is an advantage compared to other OS's. There are, however, still a few things that require to stop applications and unmount filesystems. Examples are moving a filesystem from one volume group to another, merging 2 VGs into one, ...

Commands like migratepv, reorgvg, migratelv, .. work well and online, as long as you stay in the same VG, because behind the scenes they are actually: adding an LVM mirror copy, sync'ing and removing a copy. As LVM mirroring is currently limited to having copies within the same volume group, these commands are also limited to one volume group.

If LVM mirroring across VGs was possible, then the previously mentioned commands would no longer be limited to a single volume group. Besides that, it would also enable to split or merge VGs (by mirroring LVs between the 2 VGs and removing the old copy from the old vg). Going from a big VG to a scalable VG is currently an offline activity, but with this new feature it would be possible by creating, for example, a new scalable VG and migrating all FSs to the new one online.

This will obviously be a major development effort, but also a major LVM improvement. Possibly the biggest one in years. Even if mirroring across VGs would only be possible/allowed temporarily, it would save much of the remaining planned downtime.