add build server postmortem: disk full, CI dead

root causes: concurrent=64 shell executor, LXD image
accumulation (50G), orphaned containers, zombie processes,
no disk monitoring. fixes applied on build.unturf.com:
concurrent=4, image expiry, cleanup cron, disk alerts.
recovered 70G (100% → 31%).

content/2026-03-08-build-server-postmortem-disk-full.rst

@@ -0,0 +1,134 @@
+build server postmortem: disk full, CI dead
+############################################
+
+:author: Russell Ballestrini
+:slug: build-server-postmortem-disk-full
+:date: 2026-03-08
+:tags: DevOps, Guide
+:status: published
+
+On 2026-03-07 at ~01:06 UTC, a ``git push`` to ``russell.ballestrini.net`` triggered CI pipeline #21163. It failed instantly:
+
+.. code-block:: text
+
+    bash: line 62: printf: write error: No space left on device
+
+Build server root filesystem sat at 100%. Zero bytes available. Every CI job across every project failed the same way. The build server had gone deaf.
+
+|
+
+timeline
+========
+
+- **~01:06 UTC** — Pipeline #21163 fails. ``No space left on device`` during ``get_sources``.
+- **~01:35 UTC** — SSH into ``build.unturf.com``. ``df`` confirms ``/`` at 100% (98G used, 0 available). Swap at 97%. 31 zombie processes.
+- **~01:40 UTC** — Removed 9.4G stale golden image tarball from ``/tmp``. Freed first bytes.
+- **~01:45 UTC** — Killed 15+ orphaned python3 processes from a dead ``uncloseai-cli`` CI build holding deleted directories open.
+- **~01:50 UTC** — Deleted 18 stopped LXD ``build-*`` containers (orphaned CI test containers).
+- **~01:52 UTC** — Purged 12 cached LXD images (50G total, including 9G Ubuntu server images). Disk dropped to 42%.
+- **~01:55 UTC** — Cleaned stale build slot directories (slots 4-63). Reduced ``concurrent`` from 64 to 4. Installed cleanup cron. Disk at 31%.
+- **~02:00 UTC** — Retriggered pipeline. Build passed.
+
+|
+
+root causes
+============
+
+Five independent failures conspired. Each alone stays survivable. Together they filled 98G in weeks.
+
+1. gitlab-runner concurrent = 64
+---------------------------------
+
+``/etc/gitlab-runner/config.toml`` set ``concurrent = 64``. A shell executor creates a full git checkout per slot per project. 64 slots across multiple repos meant 64 copies of every codebase. The ``unsandbox-all-upgradable`` repo alone stored 3.7G FreeBSD & 710M OpenBSD qcow2 images per slot. Slot 15 alone consumed 4.3G.
+
+**Fix applied:** ``concurrent = 4``. Matches actual workload. Stale slots 4-63 removed.
+
+2. LXD images never expired
+-----------------------------
+
+CI pipelines launch LXD containers for multi-distro testing (Alpine, Arch, Debian, Fedora, Rocky, Ubuntu, FreeBSD, OpenBSD). LXD cached every base image indefinitely. 12 images accumulated to 50G. Two Ubuntu server images weighed 9G each.
+
+``images.remote_cache_expiry`` defaulted to 10 days but never cleaned up because ``images.auto_update_interval`` kept refreshing them. No pruning mechanism existed.
+
+**Fix applied:** ``images.remote_cache_expiry = 3``, ``images.auto_update_interval = 0``. Weekly cron prunes unused images.
+
+3. orphaned LXD containers
+---------------------------
+
+CI pipelines create ``build-*`` containers but don't always clean them on failure. 18 stopped containers accumulated. Each carried a full rootfs snapshot.
+
+**Fix applied:** Hourly cron deletes stopped ``build-*`` containers.
+
+4. zombie processes holding deleted files
+------------------------------------------
+
+An ``uncloseai-cli`` build spawned python3 test servers that outlived the CI job. The runner deleted the build directory, but 15+ processes still held references to the deleted path (``cwd`` pointed to a deleted inode). These became zombies. The 31 zombie count in ``motd`` signaled this.
+
+**Fix applied:** Hourly cron kills orphaned gitlab-runner processes holding deleted directories (``lsof +L1`` detection).
+
+5. no disk monitoring
+----------------------
+
+Disk grew from comfortable to critical over weeks. No alert fired. No cron checked. Nobody noticed until CI broke.
+
+**Fix applied:** Cron checks disk every 15 minutes, logs warnings above 85% to ``/var/log/disk-alert.log``.
+
+|
+
+cleanup cron
+=============
+
+Installed at ``/etc/cron.d/build-cleanup``:
+
+.. code-block:: bash
+
+    # Delete stopped LXD build containers hourly
+    0 * * * * root lxc list --format csv -c n,s | grep STOPPED | grep '^build-' | \
+        cut -d',' -f1 | xargs -I{} lxc delete {}
+
+    # Prune unused LXD images weekly (Sunday 3am)
+    0 3 * * 0 root lxc image list --format csv -c f | tr ',' '\n' | \
+        xargs -I{} lxc image delete {}
+
+    # Clean /tmp files older than 7 days
+    0 4 * * * root find /tmp -maxdepth 1 -user gitlab-runner -mtime +7 -delete
+
+    # Kill orphaned processes holding deleted directories
+    0 * * * * root lsof +L1 | grep gitlab-runner | grep deleted | \
+        awk '{print $2}' | sort -u | xargs -r kill
+
+    # Disk alert: log warning if / exceeds 85%
+    */15 * * * * root df --output=pcent / | tail -1 | tr -d ' %' | \
+        awk '{if ($1 > 85) print strftime("[%Y-%m-%d %H:%M]"), "DISK WARNING:", $1"%"}' \
+        >> /var/log/disk-alert.log
+
+|
+
+disk recovery
+==============
+
+.. code-block:: text
+
+    Before:  98G used /  98G total = 100%  (0 bytes free)
+    After:   29G used /  98G total =  31%  (65G free)
+
+    Space recovered:
+      LXD images purged .............. 50G
+      Stale build slots removed ...... 10G
+      /tmp tarball removed ...........  9G
+      Go module cache cleaned ........  1G
+                                      ----
+      Total freed .................... 70G
+
+|
+
+lessons
+========
+
+**hardcoded limits carry hidden debt.** ``concurrent = 64`` seemed harmless on day one. By week eight, 64 slots × multiple repos × qcow2 images = full disk. Today's default becomes tomorrow's outage.
+
+**five failures don't equal five problems.** Each CI failure looked like "disk full." The actual defect graph had five nodes: excessive concurrency, image caching, container orphaning, process zombies, no monitoring. Fixing only one delays the outage. Fixing all five prevents it.
+
+**absence of signal stays a signal.** No disk alert fired because no disk alert existed. Silence in monitoring always means one of two things: everything works, or nothing watches. Assume the second until proven otherwise.
+
+**build servers need janitors.** CI systems produce waste: cached images, stopped containers, orphaned processes, stale checkouts. Without automated cleanup, waste accumulates until something breaks. The cron job costs nothing. The outage costs a pipeline.