systemd version the issue has been seen with
257.9
Used distribution
Debian 13
Linux kernel version used
6.12.57+deb13-amd64
Component
systemd-journald
Expected behaviour you didn't see
Log writes should be within order of magnitude of syslog
Unexpected behaviour you saw
VM doing ~50 IOPS when writing 2 lines of log per second
Steps to reproduce the problem
this is exactly same issue #15292 that was closed without good reason
Step 1. Use journald in mode where it writes to hard drive. The FS is XFS
Step 2. have constant stream of log entries going on a VM
Jan 03 13:37:01 cthylla haproxy[727]: 192.168.1.1:48550 [03/Jan/2026:13:37:01.392] f_www b_icinga/web 0/0/0/6/6 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:03 cthylla haproxy[727]: 192.168.1.1:36892 [03/Jan/2026:13:37:03.403] f_www b_icinga/web 0/0/0/7/7 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:05 cthylla haproxy[727]: 192.168.1.1:36904 [03/Jan/2026:13:37:05.416] f_www b_icinga/web 0/0/0/6/6 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:07 cthylla haproxy[727]: 192.168.1.1:36906 [03/Jan/2026:13:37:07.427] f_www b_icinga/web 0/0/0/6/6 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:09 cthylla haproxy[727]: 192.168.1.1:36912 [03/Jan/2026:13:37:09.439] f_www b_icinga/web 0/0/0/7/8 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:11 cthylla haproxy[727]: 192.168.1.1:36918 [03/Jan/2026:13:37:11.454] f_www b_icinga/web 0/0/0/6/6 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:13 cthylla haproxy[727]: 192.168.1.1:45832 [03/Jan/2026:13:37:13.465] f_www b_icinga/web 0/0/0/7/7 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:15 cthylla haproxy[727]: 192.168.1.1:45848 [03/Jan/2026:13:37:15.476] f_www b_icinga/web 0/0/0/6/6 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:17 cthylla haproxy[727]: 192.168.1.1:45856 [03/Jan/2026:13:37:17.488] f_www b_icinga/web 0/0/0/6/6 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Jan 03 13:37:19 cthylla haproxy[727]: 192.168.1.1:45862 [03/Jan/2026:13:37:19.500] f_www b_icinga/web 0/0/0/6/6 302 153 - - ---- 2/2/0/0/0 0/0 "GET / HTTP/1.0"
Step 3. Observe the VM IO traffic.
I used VM as example because the complaint in #15292 was "iotop is not accurate" (I can believe that, it's before any OS write coaelscing) but this clearly shows traffic after every kernel mechanism was used. So no, it isn't "kernel making lotsa iops out of it", it's slow.
Journald just uses extremely inefficient format (also I've seen it corrupt on unclean reboot enough times to declare it's not even all that resilient) as files are also multiple times the size of what's actually written in them.