tl;dr: A bug report against systemd-journald 257.9 shows that a single ~200-byte log line triggers roughly 49KB of disk writes on ext4 and 110KB on btrfs, producing ~50 IOPS for just 2 log lines/second on a VM. The reporter argues this is measured after kernel write coalescing (ruling out the prior dismissal in issue #15292), and blames journald's inefficient on-disk format, which also bloats file sizes and has proven fragile across unclean reboots.