That sounds pretty much like what Dirvish and rsnapshot do. Both wrap rsync.

Does your FS support online resizing?

Yeah. I mainly use btrfs; it supports online growing and shrinking.

Be super careful about partition sizes. […]

I know. I have done plenty of same device partition resizing. I know the pit falls, and for safety shrink the FS to below what the LV is going to be.

Have backups.

Thanks for the reminder. I’ve been meaning to set up snapshot backups for this machine using rsnapshot as an experiment. I mainly use Dirvish

Nah, it’ll be fine.

I might have a large enough USB SSD laying around some where. I could probably use that instead.

• extend VG with the USB PV
• move LV’s to USB
• shrink root LV
• resize boot
• move things back on a new internal PV

That’s not even a bad idea then.

One of my machines has a boot partition that’s a bit too small, on an otherwise LVM setup.

Good grief. Why?

jdupes is my go-to solution for file deduplication. It should be able to remove duplicate files. I don’t know how much control it gives you over which duplicate to remove though.