docs: add memory issues follow-up plan

Track zram change effectiveness for OOM prevention during upgrades.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/memory-issues-follow-up.md

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+# Memory Issues Follow-up
+
+Tracking the zram change to verify it resolves OOM issues during nixos-upgrade on low-memory hosts.
+
+## Background
+
+On 2026-02-08, ns2 (2GB RAM) experienced an OOM kill during nixos-upgrade. The Nix evaluation process consumed ~1.6GB before being killed by the kernel. ns1 (manually increased to 4GB) succeeded with the same upgrade.
+
+Root cause: 2GB RAM is insufficient for Nix flake evaluation without swap.
+
+## Fix Applied
+
+**Commit:** `1674b6a` - system: enable zram swap for all hosts
+
+**Merged:** 2026-02-08 ~12:15 UTC
+
+**Change:** Added `zramSwap.enable = true` to `system/zram.nix`, providing ~2GB compressed swap on all hosts.
+
+## Timeline
+
+| Time (UTC) | Event |
+|------------|-------|
+| 05:00:46 | ns2 nixos-upgrade OOM killed |
+| 05:01:47 | `nixos_upgrade_failed` alert fired |
+| 12:15 | zram commit merged to master |
+| 12:19 | ns2 rebooted with zram enabled |
+| 12:20 | ns1 rebooted (memory reduced to 2GB via tofu) |
+
+## Hosts Affected
+
+All 2GB VMs that run nixos-upgrade:
+- ns1, ns2 (DNS)
+- vault01
+- testvm01, testvm02, testvm03
+- kanidm01
+
+## Metrics to Monitor
+
+Check these in Grafana or via PromQL to verify the fix:
+
+### Swap availability (should be ~2GB after upgrade)
+```promql
+node_memory_SwapTotal_bytes / 1024 / 1024
+```
+
+### Swap usage during upgrades
+```promql
+(node_memory_SwapTotal_bytes - node_memory_SwapFree_bytes) / 1024 / 1024
+```
+
+### Zswap compressed bytes (active compression)
+```promql
+node_memory_Zswap_bytes / 1024 / 1024
+```
+
+### Upgrade failures (should be 0)
+```promql
+node_systemd_unit_state{name="nixos-upgrade.service", state="failed"}
+```
+
+### Memory available during upgrades
+```promql
+node_memory_MemAvailable_bytes / 1024 / 1024
+```
+
+## Verification Steps
+
+After a few days (allow auto-upgrades to run on all hosts):
+
+1. Check all hosts have swap enabled:
+   ```promql
+   node_memory_SwapTotal_bytes > 0
+   ```
+
+2. Check for any upgrade failures since the fix:
+   ```promql
+   count_over_time(ALERTS{alertname="nixos_upgrade_failed"}[7d])
+   ```
+
+3. Review if any hosts used swap during upgrades (check historical graphs)
+
+## Success Criteria
+
+- No `nixos_upgrade_failed` alerts due to OOM after 2026-02-08
+- All hosts show ~2GB swap available
+- Upgrades complete successfully on 2GB VMs
+
+## Fallback Options
+
+If zram is insufficient:
+
+1. **Increase VM memory** - Update `terraform/vms.tf` to 4GB for affected hosts
+2. **Use remote builds** - Configure `nix.buildMachines` to offload evaluation
+3. **Reduce flake size** - Split configurations to reduce evaluation memory