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The backup verifier did not fail because the PostgreSQL dump was corrupt. Each throwaway restore created an anonymous Docker volume on /mnt/data; timeout and forced-removal paths deleted the container but left that volume behind. After 34 days, 57 abandoned volumes consumed 156 GB, filled Docker’s data partition, and prevented the next PostgreSQL instance from running initdb. The durable fix was changing forced cleanup from docker rm -f to docker rm -fv, then adding orphan-volume cleanup and diagnostics for the actual Docker filesystem.
What happened to the backup verifier?
A nightly job restored a fresh PostgreSQL dump into a disposable container and ran sanity queries. The dump and storage archive completed, but the restore gate began reporting the seven-word error “postgres did not become ready.” The underlying database was not the first thing that broke: Docker’s storage partition had already reached its limit.
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| Incident detail | What the postmortem recorded |
|---|---|
| Abandoned storage | 156 GB in 57 dangling anonymous volumes |
| Accumulation period | 34 days |
| Docker data partition | /mnt/data at 100% use, with zero bytes free |
| Root partition check | / appeared to be only 61% used |
| Cleanup result | 163 GB reclaimed; /mnt/data fell to 17% use |
Those figures are measurements from the Vasyl/textstack.app incident report in 2026, not an industry-wide rate for Docker volume leaks. The cleanup reclaimed more than the measured 156 GB because other disposable Docker data was removed along with the leaked PostgreSQL volumes.
The postmortem’s central warning is blunt: “The backup broke the very step that verified the backup.” A restore test is only useful if its own temporary artifacts cannot exhaust the storage pool needed to run the test.
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The incident also shows why a successful backup job and a successful restore verifier are different checks. The dump and storage tarball succeeded; it was the restore gate that failed. As the author put it, “a backup you never restored is a hope, not a backup.” That is a first-person lesson from this incident, not an industry-wide measurement.
Why did Docker create an anonymous PostgreSQL volume?
The official PostgreSQL image declares VOLUME /var/lib/postgresql/data. If a container starts without a mount supplied for that path, Docker creates an anonymous volume and attaches it to the container. That volume holds the initialized PostgreSQL cluster, not merely a few log files, so every successful initialization can consume substantial space.
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- Two containers started from the same image can receive two different anonymous volumes.
- Removing a container does not automatically remove its anonymous volumes in the ordinary case.
- Running a container with
--rmasks Docker to remove anonymous volumes when the container exits normally. - A timeout or kill path that removes the container separately can leave the volume behind unless that removal explicitly requests volume cleanup.
The verifier normally used --rm, which covered the happy path. Its timeout handler instead ran docker rm -f. That command removed the container but did not request removal of its anonymous volumes. One initialized PostgreSQL volume was therefore left behind for each failed run.
This is why a container count alone can give a misleadingly reassuring picture: the container can be gone while its volume remains. In this incident, the leftovers were not a single oversized artifact but 57 dangling volumes accumulated over 34 days. The relevant object to track is therefore both the short-lived verifier container and the storage it caused Docker to create.
Why did df -h / say there was space when Docker could not start PostgreSQL?
Docker’s data root was on /mnt/data, a different filesystem from the root mount point. Checking only df -h / measured the wrong capacity pool. The root filesystem still had headroom, while the partition holding Docker’s layers, containers, and volumes had no free blocks.
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Any host with a separate Docker data root needs monitoring for that path, not just for the root mount. A useful check in the verifier is:
df -h /mnt/data
docker system dfThe first command reports filesystem capacity where Docker stores data. The second summarizes Docker’s reclaimable images, containers, and volumes. Run both when the job starts and whenever a container fails to become ready.
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The important diagnostic question is not simply “How full is the host?” but “How full is the filesystem Docker writes to?” A healthy result for / cannot rule out a full Docker data partition when those paths are on separate filesystems. The incident’s 61% root usage and 100% /mnt/data usage are a concrete example of that mismatch.
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How did the storage leak turn into a self-reinforcing failure loop?
The failure sequence had four linked stages. Once the data partition filled, the verifier’s own error path made the situation worse.
- Normal run: the image’s declared data volume caused Docker to create an anonymous PostgreSQL volume.
- Abnormal exit: a timeout or kill invoked
docker rm -fwithout-v, leaving that volume behind. - Capacity exhaustion: repeated failed runs accumulated 57 initialized volumes over 34 days until
/mnt/datareached 100% usage. - Readiness failure: a fresh container could not complete PostgreSQL’s
initdbwork with zero bytes available, so readiness failed. The failed verification then created another opportunity to leak a volume.
The result was not evidence that the backup dump was invalid. It was a resource failure in the disposable environment used to validate the dump.
This loop matters because the verifier depends on the same Docker storage pool that its test containers use. The job meant to check recoverability could no longer start the database needed for that check. When readiness fails, operators should therefore consider host capacity as well as the dump, PostgreSQL configuration, and readiness condition; in this case the database could not initialize because its storage filesystem was full.
What does the -v in docker rm -fv do?
Docker’s command-line reference defines -v (also written --volumes) as removing anonymous volumes associated with the container. It does not remove named volumes. Adding that flag to forced removal closes the exact gap left by the timeout handler.
| Command | Container | Anonymous volumes | Named volumes |
|---|---|---|---|
docker rm -f <container> |
Force removed | Retained | Retained |
docker rm -fv <container> |
Force removed | Removed | Retained |
Container run with --rm and normal exit |
Removed automatically | Removed as part of that cleanup | Not removed |
The one-letter change is safe only when the container’s anonymous volumes are disposable. If a container uses a named volume for state that must survive container replacement, -v does not delete that named volume; its lifecycle still requires an explicit policy.
In the incident, the change belonged in two places: the trap handler for the current verifier run and the reap of leaked verify containers left by earlier runs. Updating only the normal cleanup path would leave old containers and their volumes untouched. The fix had to prevent future leaks and clean up the existing verifier-owned leftovers.
How should a verifier guarantee cleanup on timeout and kill paths?
Use the normal --rm behavior as a convenience, but treat an explicit trap as the real safety net. The trap must cover the verifier’s success and failure paths, including timeout and forced removal.
- Keep the container ID in a variable immediately after creation.
- Register a shell trap before any operation that can fail or block.
- In the trap, call
docker rm -fvfor that container, then clear the variable so cleanup is not attempted twice. - When reaping containers left by earlier verifier processes, use the same
-vflag. - Make cleanup tolerant of an already-removed container, so a race does not hide the original verification error.
container=""
cleanup() {
if [ -n "$container" ]; then
docker rm -fv "$container" >/dev/null 2>&1 || true
container=""
fi
}
trap cleanup EXIT INT TERM
container=$(docker run -d --rm postgres:...)
# wait for readiness, run checks, and return the verifier's statusThe image tag in this example must match the version selected by your verifier; the important property is the cleanup path, not a particular PostgreSQL release. The core requirement is that the code which handles a timeout or forced stop removes the anonymous data volume as well as the container. --rm alone is not a substitute for checking the actual failure path used by the verifier.
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Should you keep anonymous volumes, switch to a named volume, or use a temporary filesystem?
There is no universal replacement. Choose according to whether each run needs isolation, how large the restore can become, and how much state you are willing to manage explicitly.
| Design | Cleanup guarantee | Orphan risk | Capacity constraint | Operational trade-off |
|---|---|---|---|---|
| Fresh anonymous volume per run | Guaranteed only when --rm and every forced path remove volumes |
High if timeout and kill handlers omit -v |
Docker data-root disk | Strong isolation, simple startup, but requires defensive cleanup |
| One named volume reused | Explicit lifecycle; container removal does not delete it | Structurally harder to orphan, but stale data can persist | Docker data-root disk | Must empty or recreate the volume before initdb; accidental reuse can invalidate a test |
| Temporary filesystem mount | Data disappears with the container | No persistent volume artifact | Available memory and the temporary filesystem’s limits | Fast and self-cleaning, but a restore larger than available memory can fail |
A named-volume design is not “set and forget.” The volume must be emptied before initialization, and the verifier must own its reset procedure. Reuse trades the per-run anonymous-volume lifecycle for explicit state management: if old PostgreSQL files remain, the next run is not starting from an empty data directory.
A temporary filesystem removes disk residue, but its capacity must be evaluated against the dump size. Its available memory constrains how much data it can hold, so it is not automatically a fit for a large restore. The right comparison is whether each choice can hold the verifier’s data, whether its state can be safely reset, and whether cleanup still works on a forced exit.
Fresh anonymous volumes offer isolation but need a reliable forced-cleanup path. A named volume can avoid creating a new anonymous volume each run, but requires deliberate emptying and lifecycle management. A temporary filesystem makes data disappear with the container, but memory capacity is the limiting cost. All three designs also need diagnostics that help explain a PostgreSQL readiness failure.
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What diagnostics should appear when PostgreSQL is not ready?
“Postgres did not become ready” is an outcome, not a diagnosis. Emit the evidence needed to distinguish a database error from an exhausted host before the container is removed.
- Print the container’s PostgreSQL logs with
docker logs <container>. - Print the filesystem containing Docker’s data root, such as
df -h /mnt/data. - Print Docker’s usage summary with
docker system df.
The incident’s readiness failure became understandable only when the error was paired with the full Docker filesystem and the retained-volume history. Logs alone describe what PostgreSQL encountered; filesystem and Docker usage show whether the container had room to initialize. Capturing all three makes the failure more actionable than a generic readiness message.
How can you clean the 57 leaked volumes without deleting important state?
Inventory first, then remove only artifacts that the verifier owns. A broad prune is powerful and therefore dangerous on a shared Docker host.
- List containers, including stopped ones, and identify old verifier containers.
- Inspect which volumes are attached to those containers before deleting anything.
- Confirm that production databases, uploads, and other retained state are bind mounts or named volumes outside the verifier’s disposable set.
- Remove stale verifier containers with
docker rm -fv. - Run
docker volume prune -fonly when your orphan-volume policy says every currently dangling volume is disposable. - Recheck
df -h /mnt/dataanddocker system df, then run a fresh restore test.
The incident used a catch-all docker volume prune -f after establishing that the dangling volumes were orphaned verification data. On a host that also runs stateful workloads, do not copy that command blindly: Docker cannot infer whether an unreferenced volume is valuable to your organization. The safety condition is an explicit policy that distinguishes disposable verification volumes from any volume whose state should be retained.
After cleanup, verify the actual filesystem rather than assuming the prune worked. In this incident, cleanup reclaimed 163 GB and moved /mnt/data from 100% to 17%. That 163 GB is the total reclaimed in this cleanup, not the amount attributed solely to the 156 GB of dangling volumes.
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How should you test the cleanup instead of trusting it?
Exercise the paths that ordinary success tests never touch. The postmortem’s key operational distinction is between normal --rm cleanup and timeout or forced-removal paths: a test that checks only a normal exit will not show whether the latter removes the anonymous volume too.
- Verify cleanup after a timeout or forced removal, not only after a normal exit.
- Check that the container’s anonymous volume is removed along with the container.
- Reap verifier containers left from earlier runs using the same volume-removal behavior.
- Confirm that readiness failures print PostgreSQL logs, Docker-filesystem usage, and Docker’s usage summary.
Track whether failed verification runs leave dangling volumes behind. The objective is not merely to get one restore through, but to ensure the verifier’s artifacts do not accumulate until they consume the filesystem it depends on.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should you monitor when Docker uses a separate data partition?
Alert on the filesystem that actually stores Docker data, with thresholds appropriate to the largest expected restore. Monitoring only host-wide root usage can miss a full Docker partition until new containers fail.
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- Used and free bytes for
/mnt/data(or the configured Docker data-root filesystem). - Docker volume count, including dangling volumes.
- Docker’s reported reclaimable space.
- Verifier success, timeout, and forced-termination outcomes.
Check both filesystem capacity and Docker’s own usage: they answer related but different questions. A rising number of dangling anonymous volumes can warn of a cleanup defect before the partition fills, while a full-filesystem check catches the immediate capacity problem. Most importantly, point the filesystem check at Docker’s actual data root, not whichever mount point is most familiar.
What does this incident teach about backup verification?
A backup that has never been restored is evidence of a successful write, not evidence that recovery works. The incident author summarized that distinction as: “a backup you never restored is a hope, not a backup.”
Verification environments need production-grade controls even when their data is disposable. The database may be temporary, but the scheduler, storage pool, cleanup code, and monitoring determine whether the test can continue to provide assurance. The safest design combines explicit volume ownership, cleanup on every exit path, monitoring of Docker’s real data filesystem, and failure messages that include enough context to act immediately.
For this verifier, the one-letter fix was not a new backup format or a PostgreSQL tuning change. It was asking forced container removal to remove its associated anonymous volumes, then ensuring the same rule applied to old verifier containers. The broader lesson is to treat cleanup as part of the restore test itself: if failure paths leave storage behind, a verifier can eventually exhaust the resource it needs to verify recovery.
Frequently Asked Questions
Why is Docker filling my disk with anonymous volumes?
An image such as the official PostgreSQL image can declare a volume path. If a container starts without a mount for that path, Docker creates an anonymous volume. These volumes can remain after ordinary container removal unless the exit or removal path explicitly cleans them up. In the textstack.app incident, timeout cleanup used docker rm -f without -v.
Why did df -h / show space when Docker said there was none?
Docker’s data root was on /mnt/data, separate from /. The root filesystem showed 61% use, while Docker’s filesystem was at 100% with zero bytes free. Check the mount that contains Docker’s data root, as well as docker system df.
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Does docker rm -f delete volumes?
It force-removes the container, but without the volume flag it can leave associated anonymous volumes behind. Use docker rm -fv when those anonymous volumes are disposable. Named volumes are retained.
What does the -v in docker rm -fv do?
Docker defines -v (or --volumes) as removing anonymous volumes associated with the container. It does not remove named volumes.
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A new PostgreSQL container needs writable space to run initdb. In this incident, Docker’s data filesystem had zero bytes free, so initialization could not complete and the verifier reported “postgres did not become ready.” The dump itself had succeeded.
How do I stop failed Docker backup tests from leaking storage?
Keep --rm for ordinary exits, but make the trap and timeout/forced-removal paths use docker rm -fv. Apply the same cleanup when reaping old verifier containers, and report PostgreSQL logs, the Docker data-root filesystem usage, and docker system df on readiness failure.
Is docker volume prune -f safe on a production host?
Only if your orphan-volume policy establishes that every dangling volume is disposable. Inventory volumes and verify ownership first; pruning is a broad cleanup operation, not a substitute for per-container lifecycle management.
Can a named volume prevent a restore test from leaking storage?
It avoids creating a fresh anonymous volume for each run if the verifier reuses a named volume, but it does not clean or reset itself. The verifier must empty it before initdb and manage its lifecycle explicitly.
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It can make data disappear with the container, but its capacity is constrained by available memory. Evaluate that limit against the dump size before adopting it.
The Bottom Line
The verifier leaked storage because timeout cleanup removed containers without removing their anonymous PostgreSQL volumes. Monitor Docker’s actual data filesystem, use docker rm -fv on forced cleanup paths, and apply prune only to volumes known to be disposable. A backup check must not exhaust the storage it needs to verify recovery.
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