If I understood you correctly, you're asking in other words: What is the difference between Volumes and bind mounts?
Differences in management and isolation on the host
Bind mounts exist on the host file system and being managed by the host maintainer.
Applications / processes outside of Docker can also modify it.
Volumes can also be implemented on the host, but Docker will manage them for us and they can not be accessed outside of Docker.
Volumes are a much wider solution
Although both solutions help us to separate the data lifecycle from containers,
by using Volumes you gain much more power and flexibility over your system.
With Volumes we can design our data effectively and decouple it from the host and other parts of the system by storing it dedicated remote locations (Cloud for example) and integrate it with external services like backups, monitoring, encryption and hardware management.
More Volumes advantages over bind mounts:
- No host concerns.
- Can be managed using Docker CLI.
- Volumes can save you some uid/gid issues related permissions which occur in cases like when a container user's uid does not match the host
gid.
- A new volume’s contents can be pre-populated by a container.
Examples
Lets take 2 scenarios.
Case 1: Web server.
We want to provide our web server a configuration file that might change frequently.
For example: exposing ports according to the current environment.
We can rebuild the image each time with the relevant setup or create 2 different images for each environment. Both of this solutions aren’t very efficient.
With Bind mounts Docker mounts the given source directory into a location inside the container.
(The original directory / file in the read-only layer inside the union file system will simply be overridden).
For example - binding a dynamic port to nginx:
version: "3.7"
services:
web:
image: nginx:alpine
volumes:
- type: bind #<-----Notice the type
source: ./mysite.template
target: /etc/nginx/conf.d/mysite.template
ports:
- "9090:8080"
environment:
- PORT=8080
command: /bin/sh -c "envsubst < /etc/nginx/conf.d/mysite.template >
/etc/nginx/conf.d/default.conf && exec nginx -g 'daemon off;'"
(*) Notice that this example could also be solved using Volumes.
Case 2 : Databases.
Docker containers do not store persistent data: any data that will be written to the writable layer in container’s union file system will be lost once the container stop running.
But what if we have a database running on a container, and the container stops - that means that all the data will be lost?
Volumes to the rescue.
Those are named file system trees which are managed for us by Docker.
For example - persisting Postgres SQL data:
services:
db:
image: postgres:latest
volumes:
- "dbdata:/var/lib/postgresql/data"
volumes:
- type: volume #<-----Notice the type
source: dbdata
target: /var/lib/postgresql/data
volumes:
dbdata:
Notice that in this case, for named volumes, the source is the name of the volume
(For anonymous volumes, this field is omitted).
