Containerization-and-DevOps

Docker Volumes

Theory: Docker Storage

By default, all files created inside a Docker container are stored in the container’s writable layer. This has two major problems:

When the container is deleted, all data is lost
Data is hard to share between containers

Docker solves this with three types of storage mounts that persist or share data outside the container lifecycle:

Host Machine
├── /var/lib/docker/volumes/   ← Named Volumes (managed by Docker)
├── /any/host/path/            ← Bind Mounts (managed by you)
└── RAM (tmpfs)                ← tmpfs Mounts (in-memory, temporary)

Types of Docker Storage

Type	Managed By	Persists After Container Stops?	Use Case
Volume	Docker	Yes	Databases, app data, sharing between containers
Bind Mount	You (host path)	Yes (it’s a host folder)	Dev workflow, config files, source code
tmpfs	OS (RAM)	No (lost on stop)	Secrets, sensitive temp data, performance

Rule: Use volumes for production data. Use bind mounts for development. Use tmpfs for sensitive/temporary data you never want on disk.

Practical

Explore docker volume Command

docker volume

Output:

Usage:  docker volume COMMAND

Manage volumes

Commands:
  create      Create a volume
  inspect     Display detailed information on one or more volumes
  ls          List volumes
  prune       Remove unused local volumes
  rm          Remove one or more volumes

Run 'docker volume COMMAND --help' for more information on a command.

Explanation:
Running docker volume with no sub-command prints the help menu listing all available volume management commands. This is useful for quickly checking what operations are available.

Create a Named Volume

docker volume create testcolume01

Output:

testcolume01

Explanation:
This creates a named volume called testcolume01. Docker stores it at /var/lib/docker/volumes/testcolume01/_data on the host. The volume name is echoed back on success.

Note: The volume name testcolume01 has a typo (should be testvolume01) but Docker accepts any valid string as a volume name — this is not an error.

List Volumes

docker volume ls

Output:

DRIVER    VOLUME NAME
local     testcolume01

Explanation:
Lists all volumes currently managed by Docker. The local driver means the volume is stored on the local host filesystem (the default). At this point only testcolume01 exists.

Write Data into a Volume

docker run -it --rm -v testvolume01:/home/app ubuntu

Inside the container:

root@2db51a71a801:/# cd /home/app
root@2db51a71a801:/home/app# echo "my sap id: 500124385" > sapid.txt
root@2db51a71a801:/home/app# cat sapid.txt
my sap id: 500124385
root@2db51a71a801:/home/app# exit

Explanation of flags:

Flag	Meaning
`-it`	Interactive terminal — attach to container’s shell
`--rm`	Automatically remove the container when it exits
`-v testvolume01:/home/app`	Mount named volume `testvolume01` at `/home/app` inside the container
`ubuntu`	Use the Ubuntu image

What happened step by step:

Docker started a temporary Ubuntu container
The volume testvolume01 was mounted at /home/app
A file sapid.txt was created with content my sap id: 500124385
The container exited and was automatically removed (--rm)
The volume and its data survived — even though the container is gone

Key insight: --rm deletes the container, NOT the volume. Volume data outlives the container.

Step 5: Verify Data Persistence Across Containers

docker run -it --rm -v testvolume01:/home/app2 ubuntu

Inside the container:

root@1f7adc478e4e:/# cd home/app2/
root@1f7adc478e4e:/home/app2# ls
sapid.txt
root@1f7adc478e4e:/home/app2# cat sapid.txt
my sap id: 500124385
root@1f7adc478e4e:/home/app2# echo testsdata > dummy.txt
root@1f7adc478e4e:/home/app2# ls
dummy.txt  sapid.txt
root@1f7adc478e4e:/home/app2# exit

Explanation:
This is a completely new, different container (note the different hostname 1f7adc478e4e vs 2db51a71a801), but the same volume testvolume01 is mounted — this time at /home/app2 instead of /home/app. The mount point name on the container side can be anything.

Results:

sapid.txt from the previous container is still there — proving data persistence
A new file dummy.txt was added — it will persist for the next container too
This also demonstrates data sharing between containers via volumes

Mount Volume as Read-Only (with -v flag)

docker run -it --rm -v testvolume01:/home/app:ro ubuntu

Inside the container:

root@4eb7a6c53ff4:/# cd home/app/
root@4eb7a6c53ff4:/home/app# ls
dummy.txt  sapid.txt
root@4eb7a6c53ff4:/home/app# cat dummy.txt
testsdata
root@4eb7a6c53ff4:/home/app# exit

Explanation:
The :ro suffix added to the -v flag makes the volume read-only inside this container. Both files from the previous steps (dummy.txt and sapid.txt) are visible and readable, but the container cannot write or modify anything in /home/app.

Syntax breakdown:

-v  testvolume01  :  /home/app  :  ro
     volume name     mount path    read-only

Use case: Mount a shared config or secrets volume as read-only for app containers so they can read but never accidentally overwrite shared data.

Mount Volume with –mount Syntax (Read-Only)

docker run -it --rm --mount type=volume,source=testvolume01,target=/home/app:ro ubuntu

Inside the container:

root@0afaebca1855:/# cd home/app:ro/
root@0afaebca1855:/home/app:ro# cd /home/app
bash: cd: /home/app: No such file or directory
root@0afaebca1855:/home/app:ro# ls
dummy.txt  sapid.txt
root@0afaebca1855:/home/app:ro# exit

Explanation:
The --mount flag is a more explicit, verbose alternative to -v. However, the :ro suffix was incorrectly placed inside the target= value, causing Docker to mount the volume at the literal path /home/app:ro instead of /home/app with read-only mode.

Wrong:

--mount type=volume,source=testvolume01,target=/home/app:ro

Correct syntax for read-only with –mount:

--mount type=volume,source=testvolume01,target=/home/app,readonly

Syntax Style	Read-Only Flag
`-v` flag	`-v volume:/path:ro`
`--mount` flag	`--mount ...,readonly`

Step 8: Bind Mount — Read-Only Directory

docker run -it --rm -v ./testdir2:/home/app:ro ubuntu

Inside the container:

root@dea7729260c1:/# cd /home/app/
root@dea7729260c1:/home/app# echo test > test
bash: test: Read-only file system
root@dea7729260c1:/home/app# exit

Explanation:
This is a bind mount — it maps a directory from the host (./testdir2) directly into the container at /home/app. The :ro flag makes it read-only, so any write attempt fails with Read-only file system.

Volume vs Bind Mount:

Named Volume:  -v testvolume01:/home/app        ← Docker manages storage location
Bind Mount:    -v ./testdir2:/home/app           ← You control the host path

Note: ./testdir2 is a relative path. Docker resolves it relative to the current working directory on the host.

tmpfs Mount

docker run -it --rm --mount type=tmpfs,target=/home/app ubuntu

Inside the container:

root@51772b4cd608:/# exit

Explanation:
A tmpfs mount stores data in the host’s RAM, not on disk. The data is available only while the container is running — once the container stops, all tmpfs data is gone permanently, with no trace on disk.

When to use tmpfs:

Storing secrets or tokens that should never touch disk
High-performance temporary file operations
Scratch space for in-memory processing

tmpfs is not for persistence — it is the opposite of a volume. Use it specifically when you want data to disappear.

Errors & Fixes Encountered

Bind Mount Source Path Does Not Exist

docker run -it --rm --mount type=bind,source=./testdir22,target=/home/app ubuntu

Error Output:

docker: Error response from daemon: invalid mount config for type "bind":
bind source path does not exist: /mnt/d/ccvt/sem 6/devops/testdir22

Cause: Unlike named volumes (which Docker creates automatically), bind mounts require the host directory to already exist. testdir22 had not been created.

Fix: Create the directory first, then run:

mkdir testdir22
docker run -it --rm --mount type=bind,source=$(pwd)/testdir22,target=/home/app ubuntu

`$(pwd)` Expansion Breaks –mount Parsing (Path with Spaces)

docker run -it --rm --mount type=bind,source=$(pwd)/testdir22,target=/home/app ubuntu

Error Output:

invalid argument "type=bind,source=/mnt/d/ccvt/sem" for "--mount" flag: target is required

Cause: The working directory path contains a space (sem 6/devops). When $(pwd) expands, the shell splits on the space — --mount receives type=bind,source=/mnt/d/ccvt/sem as one argument and 6/devops/testdir22,target=/home/app as another, breaking the parsing.

Fix: Quote the entire --mount value:

docker run -it --rm --mount "type=bind,source=$(pwd)/testdir22,target=/home/app" ubuntu

5. Key Observations

Observation	What It Proves
`sapid.txt` survived after container with `--rm` exited	Volumes persist independently of container lifecycle
Second container found `sapid.txt` without any special steps	Same volume can be shared across multiple containers
`:ro` prevented `echo test > test` from writing	Read-only mounts are enforced at the OS level
Bind mount failed because `testdir22` didn’t exist	Docker doesn’t auto-create bind mount source directories
`$(pwd)` broke with spaces in path	Paths with spaces must be quoted in shell commands
`tempfs` caused “mount type unknown”	Docker type names are case-sensitive and exact
`--mount target=/home/app:ro` created a literal `:ro` path	`:ro` is `-v` syntax only; `--mount` uses `readonly` keyword

6. Volume vs Bind Mount vs tmpfs — Comparison

Feature	Named Volume	Bind Mount	tmpfs
Storage location	Docker-managed (`/var/lib/docker/volumes/`)	Any host path you specify	Host RAM only
Created automatically	Yes	No (must exist first)	Yes (in memory)
Persists after container stops	Yes	Yes	No
Shareable between containers	Yes	Yes	No
Works on all platforms	Yes	Path issues on Windows/WSL	Yes (Linux only)
Best for	Production data, databases	Dev source code, configs	Secrets, sensitive temp data
Mount syntax	`-v volname:/path`	`-v ./hostpath:/path`	`--mount type=tmpfs,target=/path`

Quick Reference Cheatsheet

Mounting in docker run

Syntax	Description
`-v myvolume:/container/path`	Mount named volume (read-write)
`-v myvolume:/container/path:ro`	Mount named volume (read-only)
`-v ./hostdir:/container/path`	Bind mount host directory
`-v ./hostdir:/container/path:ro`	Bind mount read-only
`--mount type=volume,source=myvol,target=/path`	Named volume (–mount style)
`--mount type=volume,source=myvol,target=/path,readonly`	Named volume read-only
`--mount type=bind,source=/host/path,target=/path`	Bind mount (–mount style)
`--mount type=tmpfs,target=/path`	tmpfs in-memory mount

-v vs –mount

Scenario	Recommended Syntax
Quick single mount	`-v` (shorter)
Production / scripts	`--mount` (explicit, less error-prone)
Read-only volume	`-v vol:/path:ro` OR `--mount ...,readonly`
Path with spaces	`--mount "type=bind,source=$(pwd)/my dir,target=/app"`

WSL Command

aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker volume
Usage:  docker volume COMMAND

Manage volumes

Commands:
  create      Create a volume
  inspect     Display detailed information on one or more volumes
  ls          List volumes
  prune       Remove unused local volumes
  rm          Remove one or more volumes

Run 'docker volume COMMAND --help' for more information on a command.
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker voulme create testcolume01
docker: unknown command: docker voulme

Run 'docker --help' for more information
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker volume create testcolume01
testcolume01
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker volume ls
DRIVER    VOLUME NAME
local     testcolume01
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm -v testvolume01:/home/app ubuntu
root@2db51a71a801:/# cd /home/app
root@2db51a71a801:/home/app# echo "my sap id: 500124385" > sapid.txt
root@2db51a71a801:/home/app# cat sapid.txt
my sap id: 500124385
root@2db51a71a801:/home/app# exit
exit
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker ps -a
CONTAINER ID   IMAGE                     COMMAND                  CREATED       STATUS                     PORTS                                         NAMES
b973c8400a01   nginx                     "/docker-entrypoint.…"   3 days ago    Exited (0) 3 days ago                                                    cranky_wilbur
072d0a0360ed   sapid-checker:500124385   "python app.py"          5 days ago    Exited (0) 5 days ago                                                    amazing_shamir
c907b5626bda   sapid-checker:500124385   "python app.py"          5 days ago    Exited (1) 5 days ago                                                    eloquent_fermi
9dda3b3dd60b   sapid-checker:500124385   "python app.py"          5 days ago    Exited (0) 5 days ago                                                    wonderful_blackwell
c9d26a2f2d20   hello-c-app               "/hello"                 5 days ago    Exited (0) 5 days ago                                                    intelligent_rubin
acc3227fbfb0   my-flask-app:v1           "python /app/app.py"     6 days ago    Exited (255) 5 days ago    0.0.0.0:5000->5000/tcp, [::]:5000->5000/tcp   my-flask
3dc6fee2455d   b3/java-app:3.0           "bash"                   2 weeks ago   Exited (0) 2 weeks ago                                                   compassionate_bohr
b65389cf84cb   myrepo/java-app:1.0       "bash"                   2 weeks ago   Exited (127) 2 weeks ago                                                 reverent_swartz
b5334b76c24b   ubuntu:22.04              "bash"                   2 weeks ago   Exited (0) 2 weeks ago                                                   java_lab
0ded4732b3fb   c881927c4077              "/docker-entrypoint.…"   3 weeks ago   Exited (0) 3 weeks ago                                                   festive_hopper
81870169556e   ubuntu                    "/bin/bash"              3 weeks ago   Exited (0) 3 weeks ago                                                   dockerclass
26101d4f3ed4   ubuntu                    "/bin/bash"              3 weeks ago   Exited (0) 3 weeks ago                                                   eager_sutherland
e30f8bb4bdfe   ubuntu                    "/bin/bash"              3 weeks ago   Exited (0) 3 weeks ago                                                   eloquent_blackwell
9e0dba4e3626   c881927c4077              "/docker-entrypoint.…"   3 weeks ago   Exited (0) 3 weeks ago                                                   nginx-container
c7595ae5f6b2   c881927c4077              "/docker-entrypoint.…"   3 weeks ago   Exited (0) 3 weeks ago                                                   sharp_brahmagupta
f7a439271847   c881927c4077              "/docker-entrypoint.…"   3 weeks ago   Exited (0) 3 weeks ago                                                   kind_yalow
0cadda32442f   hello-world               "/hello"                 3 weeks ago   Exited (0) 3 weeks ago                                                   dazzling_lehmann
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm -v testvolume01:/home/app2 ubuntu
root@1f7adc478e4e:/# cd home/
root@1f7adc478e4e:/home# cd app2/
root@1f7adc478e4e:/home/app2# ls
sapid.txt
root@1f7adc478e4e:/home/app2# cat sapid.txt
my sap id: 500124385
root@1f7adc478e4e:/home/app2# echo testsdata > dummy.txt
root@1f7adc478e4e:/home/app2# ls
dummy.txt  sapid.txt
root@1f7adc478e4e:/home/app2# exit
exit
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm -v testvolume01:/home/app:ro ubuntu
root@4eb7a6c53ff4:/# cd home/app/
root@4eb7a6c53ff4:/home/app# ls
dummy.txt  sapid.txt
root@4eb7a6c53ff4:/home/app# cat dummy.txt
testsdata
root@4eb7a6c53ff4:/home/app# exit
exit
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm --mount type=volume,source=testvolume01,target=/home/app:ro ubuntu
root@0afaebca1855:/# cd home/app\:ro/
root@0afaebca1855:/home/app:ro# cd /home/app
bash: cd: /home/app: No such file or directory
root@0afaebca1855:/home/app:ro# ls
dummy.txt  sapid.txt
root@0afaebca1855:/home/app:ro# exit
exit
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ ls
container.tar  flask-docker-project  flask-lab  image.tar  java-app.tar  sapid-test
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ mkdir testdir
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ cd testdir/
aniket@Aniket:/mnt/d/ccvt/sem 6/devops/testdir$ cd ..
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm -v ./testdir2:/home/app:ro ubuntu
root@dea7729260c1:/# cd /home/app/
root@dea7729260c1:/home/app# echo test > test
bash: test: Read-only file system
root@dea7729260c1:/home/app# exit
exit
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm -mount type=bind,source=./testdir22,target=/home/app ubuntu
invalid argument "ount" for "-m, --memory" flag: invalid size: 'ount'

Usage:  docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

Run 'docker run --help' for more information
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm --mount type=bind,source=./testdir22,target=/home/app ubuntu
docker: Error response from daemon: invalid mount config for type "bind": bind source path does not exist: /mnt/d/ccvt/sem 6/devops/testdir22

Run 'docker run --help' for more information
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm --mount type=bind,source=$(pwd)/testdir22,target=/home/app ubuntu
invalid argument "type=bind,source=/mnt/d/ccvt/sem" for "--mount" flag: target is required

Usage:  docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

Run 'docker run --help' for more information
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm --mount type=tempfs,target=/home/app ubuntu
docker: Error response from daemon: invalid mount config for type "tempfs": mount type unknown

Run 'docker run --help' for more information
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker run -it --rm --mount type=tmpfs,target=/home/app ubuntu
root@51772b4cd608:/# exit
exit
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$

Containerization-and-DevOps

Docker Volumes

Theory: Docker Storage

Types of Docker Storage

Practical

Explore docker volume Command

Create a Named Volume

List Volumes

Write Data into a Volume

Step 5: Verify Data Persistence Across Containers

Mount Volume as Read-Only (with -v flag)

Mount Volume with –mount Syntax (Read-Only)

Step 8: Bind Mount — Read-Only Directory

tmpfs Mount

Errors & Fixes Encountered

Bind Mount Source Path Does Not Exist

$(pwd) Expansion Breaks –mount Parsing (Path with Spaces)

5. Key Observations

6. Volume vs Bind Mount vs tmpfs — Comparison

Quick Reference Cheatsheet

Mounting in docker run

-v vs –mount

WSL Command

`$(pwd)` Expansion Breaks –mount Parsing (Path with Spaces)