A question that regularly comes up when I talk about containers is, “can you specify where the containers/images live on the host?”

This is a good question as the install for docker is great because it’s so simple but bad because well, you don’t get many options to configure.

It makes sense that you’d want to move the containers/images off the C:\ drive for many reasons such as leaving the drive for the OS only but also, say you have a super fast SSD on your host that you want to utilise? OK, spinning up containers is quick but that doesn’t mean we can’t make it faster!

So, can you move the location of container and images on the host?

Well, yes!

There’s a switch that you can use when starting up the docker service that will allow you to specify the container/image backend. That switch is -g

Now, I’ve gone the route of not altering the existing service but creating a new one with the -g switch. Mainly because I’m testing and like rollback options but also because I found it easier to do it this way.

So the default location for containers and images is: – C:\ProgramData\docker

OK, let’s run through the commands to create a new service pointing the container/images backend to a custom location.

First we’ll create a new directory on the new drive to host the containers (I’m going to use a location on the E: drive on my host as I’m working in Azure and D: is assigned to the temporary storage drive): –

new-item E:\Containers -type directory

Now stop the existing docker service and disable it: –

stop-service Docker

set-service Docker -StartupType Disabled

get-service Docker

Now we’re going to create a new service pointing the container backend to the new location: –

new-service -name Docker2 -BinaryPathName "C:\Program Files\docker\dockerd.exe -g E:\Containers --run-service" -StartupType Automatic

Now start the new service up: –

start-service Docker2

get-service Docker2

And check the new location: –

Cool, the service has generated the required folder structure upon startup and any new images/containers will be stored here.

Once thing to mention is that if you have images and containers in the old location they won’t be available to the new service. I’ve tried copying the files and folder in C:\ProgramData\docker to the new location but keep getting access denied errors on the windowsfilter folder.

To be honest, I haven’t spent much time on that as if you want to migrate your images from the old service to the new one you can export out and then load in by following the instructions here.

Thanks for reading!

Last week I was having an issue with a SQL install within a container and to fix I needed to copy the setup log files out of the container onto the host so that I could review.

But how do you copy files out of a container?

Well, thankfully there’s the docker cp command. A really simple command that let’s you copy whatever files you need out of a running container into a specified directory on the host.

I’ll run through a quick demo but I won’t install SQL, I’ll use an existing SQL image and grab its Summary.txt file.

If you don’t have the 2017 SQL image, you can pull it from the docker hub by running: –

docker pull microsoft/mssql-server-windows

Once you have the image, execute the following to spin up a container: –

docker run -d -p 15789:1433 --env ACCEPT_EULA=Y --env sa_password=Testing11@@ --name testcontainer microsoft/mssql-server-windows

Excellent! Now we can open up a powershell session within the container: –

docker exec -it testcontainer powershell

Once we’re in we can verify where the file is: –

cd "C:\Program Files\Microsoft SQL Server\140\Setup Bootstrap\Log\"

ls

Now exit out of the powershell session within the container. What we’re going to do is copy the Summary.txt file from the container into the C:\temp directory on the host. To do this run (on the host): –

docker cp testcontainer:"C:\Program Files\Microsoft SQL Server\140\Setup Bootstrap\Log\Summary.txt" C:\temp

Cool! Now we have the file on the host and can review.

Of course this also works for copying files into a container. Say we want to copy test.txt from C:\temp on our host into C:\ in the container. We simply run: –

docker cp C:\temp\test.txt testcontainer:C:\

Nice and easy! All we need to remember is that we always specify the source directory in the cp command first.

Thanks for reading!

One of the questions that I was asked at SQL Saturday Iceland was “how can I view the filesystem within a container?”.

This is a great question as one of the things that people find off-putting about containers is their opaqueness. It’s not obvious where everything lives within the container or how we can view the files within it.

Thankfully there’s a simple docker command that allows us to open a powershell session within a container, that command is docker exec.

Let’s run through an example. Firstly I’m going to spin up a new container running an instance of SQL Server: –

docker run -d -p 15777:1433 --env ACCEPT_EULA=Y --env sa_password=Testing11@@ --name testcontainer microsoft/mssql-server-windows

Then verify that the container is running: –

docker ps

Now, to open up the powershell session inside run: –

docker exec -it testcontainer powershell

N.B. – the -i flag keeps the powershell session open, otherwise it will immediately exit. The -t flag creates a pseudo terminal, you don’t have to include it tbh but the examples I’ve seen online always include it.

Ok, we know have a powershell session within the container and we can execute commands as normal, for example: –

get-psdrive

This will allow us to explore what the filesystem is within the container and run powershell commands as needed. In the screenshot above it’s interesting to see the space available/used on the C: drive. Have a dig around within your containers and let me know if you find anything interesting 🙂

Thanks for reading!