Explain a du vs df mismatch before deleting more files

For du vs df mismatch, the safest approach is a bounded operational change, not a command pasted without context. This runbook starts with effective state, shows the smallest candidate action, and finishes by repeating the real user or system path.

TL;DR: Df reports filesystem block allocation while du walks visible names, so deleted-open files, mount overlays, snapshots, and reserved space can separate their totals. Capture a baseline with findmnt -T /var, make the reviewed change only when the evidence matches, then verify with df -hT /var and keep the rollback ready.

Audience: Linux operators comfortable reading block-device and filesystem metadata before making storage changes. This guide assumes familiarity with Linux storage and a change window appropriate to the system.

The direct answer

The gap is explained by a named allocation class and changes only when that owner releases space. That is the success condition for du vs df mismatch; command completion by itself is not enough.

The important boundary is block device, partition or volume layer, filesystem, mount namespace, and application path. Df reports filesystem block allocation while du walks visible names, so deleted-open files, mount overlays, snapshots, and reserved space can separate their totals. If an observation does not identify which side of that boundary failed, collect a narrower observation before changing state.

How the mechanism works

For du vs df mismatch, use this mental model: a path consumes space through several layers; the kernel resolves a mount, the filesystem maps names to inodes and blocks, and the storage stack maps those blocks to a device. The model prevents a common mistake—treating configuration text, control-plane acceptance, process state, and end-user behavior as the same proof.

Follow four stages:

1. Observe: identify the exact host, object, version, owner, and active configuration.
2. Interpret: write the expected result, the abnormal result, and what would remain inconclusive.
3. Change: apply one reviewed action at the narrowest layer that contradicts the baseline.
4. Verify: repeat the original path and compare the same evidence, including adjacent safety controls.

Preflight and safety boundary

Protect the best surviving copy, identify the exact device and mount, and keep an out-of-band recovery path before changing partitions, volumes, filesystems, or fstab.

Before du vs df mismatch, record UTC time, the current version or digest, the exact target, recent changes, and who owns the workload. The rollback for this runbook is: restore service capacity or fail traffic back if restart harms the workload, then fix log rotation or retention that created the deleted-open file.

Do not continue if the target identity is ambiguous, the current state cannot be saved, the only recovery session would be at risk, or the proposed command affects more objects than the brief names.

Capture the read-only baseline

Run these commands one at a time. Replace example names and addresses deliberately; do not paste production secrets into a transcript.

findmnt -T /var
 df -hT /var
sudo du -xsh /var
sudo lsof +L1 /var 2>/dev/null

Interpret the baseline before moving on:

• Expected: the gap is explained by a named allocation class and changes only when that owner releases space.
• Abnormal: restarting an unrelated process changes nothing or the difference belongs to snapshots or filesystem accounting.
• Inconclusive: missing output can also mean the wrong context, permissions, namespace, log window, binary, or target. Prove those assumptions before treating absence as health.

Save the decisive output, exit status, and timestamp. Redact credentials, customer data, private topology, tokens, and complete environment dumps.

Apply the smallest candidate change

The following is state-changing example syntax, not an instruction to run it unchanged:

sudo systemctl restart example-writer.service

For du vs df mismatch, the proposed change is acceptable only when the read-only baseline predicts its effect and the rollback is available. The key risk is: restarting a writer to release an open deleted file can interrupt requests and discard in-memory work.

Prefer an immutable artifact, validated configuration, dry-run, transaction, candidate object, or staged target when the tool supports one. Record the exact command and UTC time so later telemetry can be correlated to the change.

Verify the result from the outside in

df -hT /var
sudo lsof +L1 /var 2>/dev/null
sudo du -xsh /var

Verification for du vs df mismatch has three layers:

1. The control plane or command reports the intended effective state.
2. The process, resource, or data path reflects that state without a new pressure signal.
3. The original user-visible or dependent-system path succeeds from an independent vantage point.

If df -hT /var succeeds but the original path still fails, stop. The change may have repaired a local symptom while DNS, policy, routing, caching, dependency, or client state remains broken.

Failure branches

The baseline does not match this runbook

When restarting an unrelated process changes nothing or the difference belongs to snapshots or filesystem accounting, do not force the candidate command. Return to identity and scope, compare a healthy peer only through effective settings, and name a new falsifiable mechanism.

The change succeeds but behavior does not

A successful sudo systemctl restart example-writer.service proves that one interface accepted a request. It does not prove convergence, readiness, data compatibility, external routing, or client recovery. Re-run the same evidence at each downstream boundary.

The change makes the system worse

Execute the written rollback: restore service capacity or fail traffic back if restart harms the workload, then fix log rotation or retention that created the deleted-open file. Preserve the failed candidate and relevant logs long enough to explain the outcome; do not destroy the evidence with broad cleanup or repeated restarts.

Operator checklist

• Confirm the exact target, context, identity, version, and active owner.
• Capture the read-only baseline and one disconfirming observation.
• Label sudo systemctl restart example-writer.service as state-changing during review.
• Keep recovery access and rollback independent of the path being edited.
• Change one layer, record UTC time, and wait for its real convergence boundary.
• Verify the original path, adjacent controls, resource pressure, and persistence.
• Update the runbook when observed behavior differs from the source-reviewed model.

Investigate it in Tryssh

$ df -hT /var
Expected: the gap is explained by a named allocation class and changes only when that owner releases space.

Tryssh can preserve this evidence loop and show a state-changing command for human approval. It does not make the operator's identity, recovery access, rollback, or platform authority decisions.

Evidence and review status

This du vs df mismatch runbook was source-reviewed on 2026-07-29 against current first-party documentation. The commands are illustrative and use example targets. The page does not claim that the change was reproduced across every distribution, managed service, version, network, or workload.

Limitations and trade-offs

Restarting a writer to release an open deleted file can interrupt requests and discard in-memory work. Managed platforms may generate configuration, restrict privileges, replace local state, or expose a different control plane than the upstream project. Confirm the installed version and provider contract before applying a repair.

Search visibility is not proof of operational correctness. Treat this page as a decision aid, preserve independent recovery, and stop when the evidence contradicts its assumptions.

Related operator runbooks

Continue with mount disk by UUID Linux, find large files Linux, the Linux storage foundation guide, and the SSH hardening checklist.

Sources and further reading

• Du vs df mismatch first-party reference
• Linux filesystem documentation
• lsblk manual
• mount manual