Docker卷不保留数据

Question

Here is my docker file:

FROM ubuntu:14.04

RUN apt-key adv --keyserver keyserver.ubuntu.com --recv-keys B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
RUN echo "deb http://apt.postgresql.org/pub/repos/apt/ precise-pgdg main" > /etc/apt/sources.list.d/pgdg.list

RUN apt-get update && apt-get -y -q install python-software-properties software-properties-common \
    && apt-get -y -q install postgresql-9.3 postgresql-client-9.3 postgresql-contrib-9.3

USER postgres
RUN /etc/init.d/postgresql start \
    && psql --command "CREATE USER pguser WITH SUPERUSER PASSWORD 'pguser';" \
    && createdb -O pguser pgdb
USER root
RUN echo "host all  all    0.0.0.0/0  md5" >> /etc/postgresql/9.3/main/pg_hba.conf
RUN echo "listen_addresses='*'" >> /etc/postgresql/9.3/main/postgresql.conf
EXPOSE 5432

RUN mkdir -p /var/run/postgresql && chown -R postgres /var/run/postgresql
VOLUME  ["/etc/postgresql", "/var/log/postgresql", "/var/lib/postgresql"]
USER postgres

CMD ["/usr/lib/postgresql/9.3/bin/postgres", "-D", "/var/lib/postgresql/9.3/main", "-c", "config_file=/etc/postgresql/9.3/main/postgresql.conf"]

Here is what I did...

1. I build the docker image:

    docker build --rm=true -t my_image/postgresql:9.3 

2. Then, I create a new directory called data in my current directory and ran the following command:

    docker run -i -t -v="data:/data" -p 5432:5432 my_image/postgresql:9.3 

3. I open another terminal and enter the postgres shell by running:

    psql -h my_docker_ip -p 5432 -U pguser -W pgdb 

   and I create a table:

    pgdb=# create table test (test_id bigserial primary key); 

4. I verify the table exist using \\dt and exit the postgres shell

5. I terminate the docker process and rerun the following:

    docker run -i -t -v="data:/data" -p 5432:5432 my_image/postgresql:9.3 

6. I enter the posgrest shell again and run \\dt

7. I notice
   • there are no tables.
   • in the data directory there are no files.

I must be doing something wrong since I am assuming that the table I created will persist. Can someone point out my mistake?

Answer 1

Based on your comment:

the data persisted, but I still can't find the persist data in my host ./data directory

and running this command:

docker run -i -t -v="data:/data" -p 5432:5432 my_image/postgresql:9.3

You appear to be confusing a named volume and a host volume. The named volume is used when you give the volume a name without a path, like data . The named volume stores the data using the docker driver (typically local) under a given name that you can reuse. It has the advantage of being listed in docker volume ls , and being initialized to the content of the image at the mounted location.

If you include a full path, like /home/username/data that would mount the directory from the docker host instead of using the named volume. The biggest disadvantage is that you don't get the directory initialized with the contents from the image, and you will likely encounter permission issues where the uid of the container process won't match the uid you use on your host.

For more details, see https://docs.docker.com/engine/tutorials/dockervolumes/

Answer 2

There is something that confused me and for me was not very clear in the official documentation.

To my knowledge, persistent volumes can be created in three ways.

• At container invocation time including full path ( -v ~/database:/data ): makes an external folder from the host available inside the docker container. Both can modify it.
• At container invocation time using a volume name ( -v datamysql:/data ): makes a volume that is persistent available inside the container. It is created it if it did not exist. You can list them by name with docker volume ls . Internally, it will be stored in a place such as /var/lib/docker/volumes/ae4445f7c9317a22fe84726fb894c47754f38a7fd150c00fd877024889968750/_data .
• At container build time ( VOLUME ["/database/data"] in Dockerfile). Every invocation of docker run will create a new volume that will persist even if you delete the container. This can be confusing becausee subsequent invocations will result in different volumes being created that will not be reused.

You can list both named (second case) and unnamed (third case) volumes with

$ docker volume ls                                                             
DRIVER              VOLUME NAME                                                             
local               064593b3e65977097d4d0c8402a6c633f1af69be2937bf118678ab8f97ee9a7e               
local               4753ad0437d13e54c76d9c34a30a1843396a1866a0cf9237d500fdcca0d78c5f           
local               8d7a35354f666b2e8a26866a35bbae36bb9601701d4c6b505ab8ce6629f69415               
local               db48eefe8f189b36107ca9c4eebb792690590ab0ba055e7e4e2c9adfd1765b7e                    
local               datamysql

You can see the exact location of a container's volume by using docker inspect mycontainer

{
                "Type": "volume",
                "Name": "8d7a35354f666b2e8a26866a35bbae36bb9601701d4c6b505ab8ce6629f69415",
                "Source": "/media/USBdrive/docker/volumes/8d7a35354f666b2e8a26866a35bbae36bb9601701d4c6b505ab8ce6629f69415/_data",
                "Destination": "/var/lib/mysql",
                "Driver": "local",
                "Mode": "",
                "RW": true,
                "Propagation": ""
            },

It might be handy to remove unused volumes (for the third case, specially).

$ docker volume prune
WARNING! This will remove all volumes not used by at least one container.
Are you sure you want to continue? [y/N] y
Deleted Volumes:
4753ad0437d13e54c76d9c34a30a1843396a1866a0cf9237d500fdcca0d78c5f

Total reclaimed space: 205MB

Because you used the VOLUME directive in your Dockerfile, you are in the third case. Inspect your container to look for the file, and specify the volume from the command line if you want repeated sessions to persist data.