Comment # 3 on bug 960910 from Vincent Petry

That's good news!

Please note that in my case GRUB is installed on the boot sector of the root
partition, not the MBR. So not sure if this is directly related.

I tried manually running /etc/cron.weekly/btrfs-trim.sh as root, and after
trimming "/" (the root partition, mounted from /dev/sda3), the boot sector was
still looking ok. No zeroes have been written there.

Maybe trim/discard only does that in more specific conditions ?

Anyway, looking forward for the fix :-)