Volumes and Persistent Storage

Overview

Persist data with named volumes, use bind mounts for live code, know when tmpfs fits, and back up a volume.

Container filesystems are ephemeral. Volumes survive container removal; bind mounts map host paths; tmpfs keeps data in memory.

This is a core tutorial in Module 7 · Volumes & Storage of the REBASH Academy Docker for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

• Dockerfile Best Practices and Multi-Stage Builds

Learning Objectives

By the end of this tutorial, you will be able to:

• Create and mount a named volume
• Contrast volume vs bind mount
• Use tmpfs for scratch data
• Backup/restore a volume with a helper container

Architecture

This topic’s control points and relationships are shown below.

Theory

What

Containers are ephemeral by default: the writable layer disappears when the container is removed. Volumes, bind mounts, and tmpfs mounts persist or isolate data differently. Named volumes are managed by Docker; bind mounts map host paths; tmpfs keeps data in memory.

Why

Databases, queues, and CI caches need durable or shared storage. Bind-mounting source code enables live development. Choosing the wrong mount type causes permission errors, data loss on recreate, or accidental exposure of host files.

How it works

Declare mounts with docker run -v / --mount or Compose volumes:. Named volumes live under Docker’s volume directory and survive container recreation. Bind mounts use an absolute host path — ideal for code, risky for production config if paths differ. tmpfs is useful for scratch space and sensitive temporary files that must not touch disk. Volume drivers can back remote storage; local is the default. Permissions inside the container depend on user IDs (UID/GID) matching ownership on the volume.

Type	Use
Named volume	Databases, durable app data
Bind mount	Dev source, host configs
tmpfs	Secrets scratch, caches (lost on stop)

Key concepts

• Lifecycle — volumes persist after docker rm unless removed explicitly
• UID mapping — non-root containers often need chowned volumes
• Backup — treat volumes as stateful; plan snapshots
• Compose namespacing — project prefixes on volume names

Common pitfalls

• Storing production data only in the container writable layer
• Bind-mounting /var/run/docker.sock without understanding host takeover risk
• Permission denied after switching USER in the image
• Deleting volumes with docker volume prune without checking labels

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-docker/module-07/host-data && cd ~/rebash-docker/module-07/host-data

Focus: persist data with a named volume across container recreate

Step 1 – Write then recreate

docker volume create rebash-data
docker run --rm -v rebash-data:/data alpine:3.20 sh -c 'echo persisted > /data/note.txt'
docker run --rm -v rebash-data:/data alpine:3.20 cat /data/note.txt
docker volume inspect rebash-data --format '{{.Mountpoint}}'

Step 2 – Remove volume

docker volume rm rebash-data
docker volume ls | grep rebash || echo "volume removed"

Final step – Cleanup note

docker volume rm rebash-data 2>/dev/null || true
# Keep ~/rebash-docker/ for later tutorials

Validation

• Lab commands run under ~/rebash-docker/module-07/host-data/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Volumes and Persistent Storage always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for docker as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

Storing production data only in the container writable layer

Validate assumptions against the Theory section and official docs before changing production.

Bind-mounting /var/run/docker.sock without understanding host takeover risk

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Volumes and Persistent Storage changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary

Volumes and Persistent Storage is essential for Cloud and DevOps engineers working with docker. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

1. Named volume versus bind mount trade-offs?
2. What happens to a named volume on docker rm?
3. How do permissions problems show up with mounts?
4. Backup approach for volume data?
5. Security risks of bind-mounting docker.sock?

Sample answer — question 2

Confirm the mount in docker inspect and file paths inside the container.

Sample answer — question 4

Never mount docker.sock into untrusted containers.

Related Tutorials

• Course overview
• Docker Networking Fundamentals

References

• Manage data in Docker