I was just going through this tutorial on Youtube, trying to understand the use of the -v command.
Why is the author using the -v command? He uses the command, like so:
docker run -v /var/lib/mysql --name=my_datastore -d busybox echo "my datastore"
Now I understand the above command to an extent:
--name=my_datastore gives the container a specific name.
-d busybox starts a container in detached mode, based on the busybox image.
After the command is executed the below line is echoed to the console.
my datastore
Now, the part I don't understand is the following:
-v /var/lib/mysql
Why is the -v command used here, and why the path /var/lib/mysql specified?
I am having difficulty understanding why the above line is used and in what context. Can anybody explain?
The -v (or --volume) argument to docker run is for creating storage space inside a container that is separate from the rest of the container filesystem. There are two forms of the command.
When given a single argument, like -v /var/lib/mysql, this allocates space from Docker and mounts it at the given location. This is primarily a way of allocating storage from Docker that is distinct from your service container. For example, you may want to run a newer version of a database application, which involves tearing down your existing MySQL container and starting a new one. You want your data to survive this process, so you store it in a volume that can be accessed by your database container.
When given two arguments (host_path:container_path), like -v /data/mysql:/var/lib/mysql, this mounts the specified directory on the host inside the container at the specified path (and, to be accurate, this can also be used to expose host files inside the container; for example -v /etc/localtime:/etc/localtime would make /etc/localtime on the host available as /etc/localtime inside the container). This is a way of either feeding information into your container, or providing a way for your container to make files accessible to the host.
If a container has volumes available, either through the use of the -v command line argument or via the VOLUME directive in a Dockerfile, these volumes can be accessed from another container using the --volumes-from option:
docker run --volumes-from my_datastore ...
This will make any volumes defined in the source container available in the container you're starting with --volumes-from.
This is discussed in more detail in the Docker Volumes documentation.
Related
When I created my installation like this:
sudo docker run -d --name myBlog -p 3001:2368 -e url=http://xxxx.com -v /var/www/myBlog/:/var/lib/ghost --restart always ghost
I was under the impression that I was defining that what ever is on /var/lib/ghost would be available in /var/www/myBlog/ but it seems that's not the case, when I check on /var/www/myBlog/ there is an empty folder. I created a new post so it can have some data but nothing is there.
Where exactly is the data being stored then? And is there a way I can access my current image that I'm using to see the files inside? I tried sudo docker run -it ghost but that gets me to the base image, not the one I'm using.
Please run the below command and you should be able to see files under target directory
docker run -d -e url=http://xxxx.com --name some-ghost -p 3001:2368 -v "$(pwd)/target":/var/lib/ghost/content ghost:1-alpine
Volumes work the other way around. The source is mapped into the container, mounted on top of whatever exists in that directory inside the container. When you mount over an existing directory in Linux, the parent filesystem at that location is no longer visible.
That means you'll see the host directory contents inside the container. Note this also includes filesystem permissions, and uid/gid ownership.
Named volumes (not the host volume used here) also feature an initialization step for an empty volume when the container is created, copying image contents to the volume before mounting it. It will not modify an existing volume.
I'm learning docker and reading their chapter "Manage data in containers". In the "Mount a host directory as a data volume". They mentioned the following paragraph:
In addition to creating a volume using the -v flag you can also mount a directory from your Docker engine’s host into a container.
$ docker run -d -P --name web -v /src/webapp:/opt/webapp training/webapp python app.py
This command mounts the host directory, /src/webapp, into the container at /opt/webapp. If the path /opt/webapp already exists inside the container’s image, the /src/webapp mount overlays but does not remove the pre-existing content. Once the mount is removed, the content is accessible again. This is consistent with the expected behavior of the mount command.
Experiment 1
Then when I tried to run this command and try to inspect the container, I found that that actually container doesn't even run. Then I use docker logs web and find this error:
can't open file 'app.py': [Errno 2] No such file or directory
I assume that the /src/webapp mount overlays on the /opt/webapp, which there is no content.
Question 1
How can I remove this mount and check if the content is still there as the quote said?
Experiment 2
When I tried to run
$ docker run -d -P --name web2 -v newvolume:/opt/webapp training/webapp python app.py
I found that the container ran correctly. Then I use docker exec -it web2 /bin/bash and find that all of the existing content are still inside the /opt/webapp. I can also add more files inside here. So in this case, it looks like that the volume is not overlay but combined. If I use docker inspect web and check Mounts, then I'll see that the volume is created under /var/lib/docker/volumes/newvolume/_data
Question 2
If I give a name instead of a host-dir absolute path, then the volume will not overlay the container-dir /opt/webapp but connect the two dir together?
An alternative solution is to commit the container (or export it) using docker cli and re-create it without doing the mapping.
Question 1 How can I remove this mount and check if the content is still there as the quote said?
You would create a new container without the volume mount. E.g.
$ docker run -d -P --name web training/webapp python app.py
(Theoretically it's possible to perform some privileged operations to remove the mount on a running container, but inside the container you will not normally have this permission, and it's a good practice to get into the habit of treating containers as ephemeral.)
Question 2 If I give a name instead of a host-dir absolute path, then the volume will not overlay the container-dir /opt/webapp but connect the two dir together?
Almost. What's happening with named volumes is that docker provides an initialization step when the volume is empty and the container is created with that volume mount. The initialization step copies the contents of the image at that directory into the volume, including all files and directories recursively, ownership, and permissions. This is very useful to running containers as a non-root user with a volume directory that the user inside the container needs to be able to write into. After that initialization has happened, future containers with the same named volume will skip the initialization, even if the image content has changed, e.g. if you add new content into the image.
I think that my question is simple but I want to make sure I am taking the right approach. On my host computer, I have a path, e.g. /my/docs, which contains HTML files which get updated automatically.
I have a Docker container with a small web server these html files. I would like to create a named coker volume to called my-docs to point to /my/docs, so that I can start the container as docker run -v my-docs:/public ...
Is this the right approach, and if so, what is the docker create volume command?
The default "local" driver for named volumes places the data inside of the docker directories. The correct way to do what you want is to use the built in host volume, instead of a named volume:
docker run -v /my/docs:/public ...
Alternatively, you can first copy the contents from /my/docs into the named volume and then use that named volume:
docker run --rm \
-v /my/docs:/source -v my-docs:/target \
busybox cp -av /source/. /target/
docker run --rm -it -v my-docs:/public busybox /bin/sh
It's also possible that someone has created a driver for this, or for you to create one yourself. See the volume driver plugin docs for more details.
I have a Docker container which is running some code and creating some HTML reports. I want these reports to be published into a specific directory on the host machine, i.e. at /usr/share/nginx/reports
The way I have gone about doing this is to mount this host directory as a data volume, i.e. docker run -v /usr/share/nginx/reports --name my-container com.containers/my-container
However, when I ssh into the host machine, and check the contents of the directory /usr/share/nginx/reports, I don't see any of the report data there.
Am I doing something wrong?
The host machine is an Ubuntu server, and the Docker container is also Ubuntu, no boot2docker weirdness going on here.
From "Managing data in containers", mounting a host folder to a container would be:
docker run -v /Users/<path>:/<container path>
(see "Use volume")
Using only -v /usr/share/nginx/reports would declare the internal container path /usr/share/nginx/reports as a volume, but would have nothing to do with the host folder.
This is one of the type of mounts available:
The answer to this question is problematic because it varies depending on your operating system and your full requirements. The answer by VonC makes some assumptions that should be addressed and is therefore only correct in some contexts. Other answers on this topic generally ignore the fact that some people are running linux, others windows, and still others are on OSX or other weird OS's.
As VonC mentioned in his answer, in a lot of cases it is possible to bind-mount a host directory straight into the container, using a -v host-path:container-path argument to the docker command (you can also use --volume for added readability or --mount for rocket-science).
One of the biggest problems (in 2020) is the use of the Windows Subsystem for Linux (WSL), where bind-mounting a host volume is fraught with error and may or may not work as expected depending on whether the path mounted is in the linux filesystem or the windows filesystem. VonC's answer was written before WSL became a big problem, but it still makes assumptions about the local filesystem being real rather than mounted into a virtual-machine of some kind.
I have found that a lot of engineers prefer to bypass this unnecessary confusion through the use of docker volumes. A docker volume can be created with the command:
docker volume create <name>
Listed with
docker volume ls
and removed with
docker volume rm <name>
You can mount this by specifying the name of the volume on the left-hand-side of the --volume argument. If your volume was called, for example, 'logs', you could use something like --volume logs:/usr/share/nginx/reports to bind it to the log dir you're interested in. You can view the contents of the directory with something like this:
docker run -it --rm --volume logs:/logs alpine ls -AlF /logs/
This should list the files in that directory. If you have a file called 'nginx.log' for example, you could view it like this:
docker run -it --rm --volume logs:/logs alpine less /logs/nginx.log
And the contents would be paged to your terminal.
You can bind this volume to multiple containers simultaneously if needed. This is useful if, for example, you're writing to your logs with one container, and paging them to a console with another.
If you want to copy the example log file from above into a tmp directory on your local filesystem you can achieve that with:
docker run -it --rm --volume logs:/logs --volume /tmp:/local_tmp alpine cp /logs/nginx.log /local_tmp/
I am using Docker toolbox on windows. I am Working on a Spring Boot Application using Docker. My application writes logs to
users/path/service.log
So when i started my application from host terminal the Log file was successfully updated.
But the same when i did on docker no file was created and neither updated.
So i changed my log file location to match with the Container's Directories
var/log/service.log
I started my container again and my file was updated again.
You can choose any location as long as it matches with the container Directory. Just bash into the container and see what suits you.
Next step is to copy log files from container to host.
So in order to copy those logs to your host. You can use one of two ways i know of-
1- use Volumes in docker
2- use following Docker command to copy file from docker container to host-:
docker cp <containerId>:/file/path/within/container /host/path/target
First, you need to create a directory where you want to share the data
mkdir -p /abc/def/
Now, you need to create a docker volume using the below command. As we see here, we are specifying device as '/abc/def/'
docker volume create --driver local \
--opt type=none \
--opt device=/abc/def/ \
--opt o=bind \
spark-volume
Now, start your container with below command..
docker run -d \
--mount type=volume,dst=/abc/def/,volume-driver=local,volume-opt=type=none,volume-opt=o=bind,volume-opt=device=/opt/spark/ \
--network host \
img:tag
Now, docker container will use /abc/def/ in local Filesystem as its storage and you will have all contents of /abc/def/ in docker container available in Local Filesystem
In your application, if you set a working directory for your php code (report path), the path must be the one on the container. Then docker will copie automaticly copy to your host directory. It wasn't docker mis-configuration, but my application that was writing to the wrong place. Weird at first, but did work in my case.
I'm slowly working my way through understanding current Docker practices. I'm on a Mac, and I'm using boot2docker.
I've been able to use the docker -v local/directory:container/directory method to link a container directory to my local file system. Great, now I can easily edit things like site code in my local Mac file system and have the changes immediately available to my container (e.g. /var/www/html).
I'm now trying to separate my containers into discrete concerns. For example, a Web, Database, and File (e.g. busybox) container would be useful for a Wordpress site. Thing is, I don't know how to make my file container define volumes that I can then link to my local OS (similar to the -v local/directory:container/directory used by boot2docker).
This is probably not the most eloquent question, as I'm still fumbling through learning Docker, but if you can understand what I'm trying to achieve, I'd really appreciate any guidance provided.
Thanks!
Docker Volumes User Guide
I will use two docker containers for my simple example
marginalized_liskov and plagiarized_engelbart
Mount a Host Directory as a Data Volume (at runtime)
docker run -d -P --name marginalized_liskov -v /host/directory/context:/container/directory/context poop python server.py
marginalized_liskov is the name of the container.
poop is not only my favorite palindrome, but also the name of the volume that we're creating.
"/host/directory/context" is the location on the host that you want to mount
"/container/directory/context" is the location you want your new volume to be created in your container
python is of course the application to run
server.py is the argument provided to "python" for this sample.
Create a Named Volume in a container and mount another container to that volume
docker create -v /poop --name marginalized_liskov training/postgres
docker run -d --volumes-from marginalized_liskov --name plagiarized_engelbart ubuntu
This creates two containers.
marginalized_liskov gets a volume created named poop I built it from the postgres training image because that's what was used in the User Guide. Since we're just setting up a container to contain a data volume and not host applications, using the training/postgres image provides our functionality while remaining lean.
plagiarized_engelbart mounts the volumes from marginalized_liskov with the --volumes-from flag.