"The server must be shut down cleanly before running pg_checksums."
looking around, i think that michael's pg_checksums_ext still seems to be available in debian repos, and it advertises online checks, which makes me think that the core pg_checksum might still not have merged any code for online checking yet
but i wanted to confirm with someone - my easiest options for online verifying checksums would be to trigger pg_basebackup then check pg_stat_database, or to install michael's utility?