The memory spike that left no trace in the logs"

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A memory spike showed up on a Grafana dashboard for a small lab I run. I checked the logs. They said nothing happened.

That "nothing" turned out to be the whole clue.

This is a short detective story about network observability — and about why "just check the logs" is only half the job. I got the first two guesses wrong, which is the interesting part, so I'll leave the mistakes in.

The setup

The lab is four Arista cEOS routers plus a couple of Alpine L2 switches, wired up with containerlab and instrumented end to end:

• routers → gNMI streaming telemetry (OpenConfig + EOS-native), collected by gnmic
• switches → SNMP via snmp_exporter
• everything into Prometheus, visualized in Grafana

The spike itself was modest — about +130 MB, gone in 90 seconds. Easy to ignore. But it was a perfect excuse to test the thing I actually care about: when a graph twitches, can the stack tell you why?

Act 1 — First guess (wrong)

The memory metric moved on all four routers in lockstep. First clue: in cEOS, /system/memory reports the underlying host, not the individual device — so a "device memory spike" is often really a host-level event, and the four numbers move together because they're all reading the same VM.

I pulled per-process memory (I'd wired up gNMI's EOS-native /Kernel/proc tree, joined to process names in PromQL) and did a quick before/after comparison. It showed a burst of short-lived systemctl processes. Case closed, I said.

I was wrong.

Act 2 — The rabbit hole

Chasing those systemctl processes led somewhere I didn't expect. In /var/log/messages:

rsyslog.service: Scheduled restart job, restart counter is at 2478505
rsyslog.service: Main process exited, code=exited, status=1/FAILURE
rsyslog.service: Failed with result 'exit-code'.

rsyslog was crash-looping — 2.4 million restarts. Root cause, once I dug in:

$ ss -lxp | grep journal/syslog
u_dgr UNCONN ... /run/systemd/journal/syslog ... users:(("systemd",pid=1,fd=88))

systemd (PID 1) holds /run/systemd/journal/syslog as a socket-activation listener. rsyslog's imuxsock wasn't consuming the passed fd — it tried to bind the socket itself, got EADDRINUSE, and exited. And the restart limiter was disabled:

Restart=on-failure
RestartUSec=100ms
StartLimitIntervalUSec=0   # rate limiting OFF -> never gives up
NRestarts=2490662

So it looped every 100 ms, forever. Completely harmless — EOS logs through its own database, not this rsyslog — and nobody had ever noticed, because you only see it if you run long enough and watch at the process level. A great bonus find.

But it was constant background noise, not the cause of this spike. Back to 13:52.

Act 3 — What the logs said: nothing

I checked every log layer, on all four devices, at the moment of the spike:

Layer	Command	Result at spike
EOS agent log	show logging	no events
ConfigAgent	/var/log/agents/ConfigAgent-*	only the boot-init line
Sysdb	/var/log/agents/Sysdb-*	last entry days earlier
Linux / systemd	/var/log/messages	only rsyslog noise, 0 other lines

Zero events. Nothing logged at all.

And that "nothing" is itself a strong signal: no config change (those log %SYS-5-CONFIG_I), no state event, no OS event. Whatever moved the memory did so silently.

Act 4 — The time series told the real story

Back to the metrics, but with discipline this time — full time series instead of a two-point comparison. The reliable signal wasn't systemctl at all; that count was inflated by stale series (a metric-expiry artifact). It was ConfigAgent + Sysdb rising together:

13:22 – 13:50   3182 MB   flat for 28 minutes
13:52           3236 MB   +54 MB, one sharp jump
13:54 – 14:36   3192 → 3183 MB   slow decay, never repeats

+54 MB, simultaneously, across all four routers. Flat before, one jump, slow decay, one-off in three hours.

Silent. Simultaneous. One-off. That shape points to one thing: a read — something querying all four devices at once. Because here's the thing:

Config writes get logged. Config reads don't. There's an entire class of activity your logs will never show you.

Act 5 — Then I proved it

A hypothesis you can't reproduce is just a story. So I ran a controlled experiment — record baseline, fire the command at all four in parallel, watch the metric.

Attempt 1 — a CLI read:

for r in 1 2 3; do for d in core1 core2 edge1 edge2; do
  docker exec clab-campus-$d Cli -c "show running-config all" &
done; done; wait

Result: 3183 → 3183 MB. No movement. Falsified. show running-config is served by the CLI process itself; it never makes ConfigAgent/Sysdb allocate.

Attempt 2 — a gNMI read:

for r in 1 2 3; do for d in core1 core2 edge1 edge2; do
  gnmic -a clab-campus-$d:6030 -u admin -p admin --insecure \
    get --path 'eos_native:/Sysdb' &
done; done; wait

Result: 4080 → 6520 MB, then a slow decay back toward baseline. Same shape as the original event. Confirmed. (The magnitude is much bigger only because I read the entire /Sysdb tree; the original was a small subtree.)

Root cause & mechanism

The 13:52 spike was a bulk gNMI read hitting all four devices — almost certainly one of my own commands while building the telemetry.

• CLI reads are served by the Cli process; the output lives in its memory → the agents' RSS never moves.
• gNMI reads go through Octa → Sysdb, serializing the state tree into gNMI/protobuf buffers. Those agents use memory pools — they don't hand memory back to the OS immediately, they GC lazily. Hence the signature: sharp jump, slow decay, settles slightly above baseline.
• Empty logs, because read operations don't log. Only writes do.

Four log layers said "nothing happened." The metrics said otherwise. Both were right — the event simply lived in the blind spot of one of them.

What I kept

• Reads leave no trace — only writes log. Rely on logs alone and a whole class of activity is invisible. You need metrics and logs and elimination.
• Two-point comparisons lie. Read the time series. My first wrong answer came from two samples; the truth was in the shape over time.
• Know what your metric actually measures. "Device memory" was host memory; a process count was inflated by expiry settings.
• A hypothesis isn't a diagnosis until you reproduce it. The experiment killed my convenient first theory and confirmed the real one.
• Deep, long-running instrumentation surfaces things nobody looks for — like a service that had quietly failed two million times.

None of this needed a fancy tool. It needed the willingness to be wrong on the first guess and to keep pulling the thread until the graph and the experiment agreed.

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The whole lab and a longer bilingual write-up are open source, if you want to poke at it or reproduce the experiment:
https://github.com/kedicat308/Arista-LLM

What's the sneakiest "the logs said nothing" bug you've run into? I'd love to hear it.