# btrfs-sync --verbose --delete /home/user/.snapshots user@server:/media/USBdrive/home-snapshots
* Skip existing '/home/user/.snapshots/monthly_2018-01-09_200102'
* Skip existing '/home/user/.snapshots/monthly_2018-02-08_200102'
* Skip existing '/home/user/.snapshots/weekly_2018-02-09_140102'
* Skip existing '/home/user/.snapshots/weekly_2018-02-16_150102'
* Skip existing '/home/user/.snapshots/weekly_2018-02-23_150102'
* Skip existing '/home/user/.snapshots/weekly_2018-03-02_180102'
* Skip existing '/home/user/.snapshots/daily_2018-03-03_000101'
* Skip existing '/home/user/.snapshots/daily_2018-03-04_080101'
* Skip existing '/home/user/.snapshots/daily_2018-03-05_100102'
* Skip existing '/home/user/.snapshots/daily_2018-03-06_100102'
* Skip existing '/home/user/.snapshots/daily_2018-03-07_110102'
* Synchronizing '/home/user/.snapshots/hourly_2018-03-08_090101' using seed '.snapshots/hourly_2018-03-07_090101'...
time elapsed [0:00:24] | rate [11.1MiB/s] | total size [ 132MiB]
* Synchronizing '/home/user/.snapshots/hourly_2018-03-08_100101' using seed '.snapshots/hourly_2018-03-09_090101'...
time elapsed [0:01:05] | rate [11.1MiB/s] | total size [ 275MiB]
* Deleting non existent snapshots...
Delete subvolume (no-commit): '/media/USBdrive/home-snapshots/hourly_2018-03-08_090101'
Delete subvolume (no-commit): '/media/USBdrive/home-snapshots/hourly_2018-03-08_100101'

Usage:
  btrfs-sync [options] <src> [<src>...] [[user@]host:]<dir>

  -k|--keep NUM     keep only last <NUM> sync'ed snapshots
  -d|--delete       delete snapshots in <dst> that don't exist in <src>
  -z|--xz           use xz     compression. Saves bandwidth, but uses one CPU
  -Z|--pbzip2       use pbzip2 compression. Saves bandwidth, but uses all CPUs
  -q|--quiet        don't display progress
  -v|--verbose      display more information
  -h|--help         show usage

<src> can either be a single snapshot, or a folder containing snapshots
<user> requires privileged permissions at <host> for the 'btrfs' command

# btrfs-sync /home/user/.snapshots /media/USBdrive/home-snapshots

# btrfs-sync /home/user/.snapshots user@server:/media/USBdrive/home-snapshots

# btrfs-sync /home/user/.snapshots/monthly_2018-02-08_200102 user@server:/media/USBdrive/home-snapshots

# btrfs-sync /home/user/.snapshots/monthly_* user@server:/media/USBdrive/home-snapshots

cat > /etc/cron.daily/btrfs-sync <<EOF
#!/bin/bash
/usr/local/sbin/btrfs-sync --quiet --keep 50 --xz /home user@host:/path/to/snaps
EOF
chmod +x /etc/cron.daily/btrfs-sync

cat > /etc/cron.daily/btrfs-sync <<EOF
#!/bin/bash
/usr/local/sbin/btrfs-sync --quiet --delete /home user@host:/path/to/snaps
EOF
chmod +x /etc/cron.daily/btrfs-sync