Can I set up a container volume from the EBS volume on the host EC2 instance?

Rack (Version 2)
1

Hello,

I’ve been profiling some of my background jobs, and I discovered that an S3 download step is taking up a large percentage of the time (between 20% and 50%.) The same files are being used repeatedly, so I would like to start caching these locally.

It would be easy to set up an ephemeral cache inside the Docker containers, but then this would be blown away every time a container restarts. Also each container would need to maintain their own cache, so this would not be an efficient use of disk space.

I’m reading the EFS FAQ:

Q: When should I use Amazon EFS vs. Amazon S3 vs. Amazon Elastic Block Store (EBS)?
…
Amazon EBS can deliver performance for workloads that require the lowest-latency access to data from a single EC2 instance.

As far as I understand, Convox uses EFS for any volumes defined in convox.yml. (At least, for version 2 racks.)

I think it might be better if I could mount a local volume from each EC2 instance (from the EBS volume), because the performance would be much better. Each set of containers on a single instance would share the same volume. For my case, it’s totally fine if a container on instance A doesn’t see the same files as a container on instance B.

One advantage of an EFS volume is that the file cache would be shared across all of my EC2 instances. I might need to be careful about lock contention, since every process would be accessing and updating the same cache. (But I probably won’t need to worry about that for a long time.)

Is there something I can modify in convox.yml, or maybe in a custom AMI?

2

I just read through the “AWS EFS — The Container Friendly File System” blog post, and that explained a lot! I actually didn’t realize that Convox was already mounting an EFS volume at /volumes, so I already have a directory that is shared between all of the instances.

So if I use this format:

  volumes:
    - /var/lib/postgresql/data

Then Docker will create a new volume inside /volumes by default.

So if I specify an explicit path outside of the /volumes directory, then I guess this means I’m using EBS instead of EFS. E.g. something like:

  volumes:
    - /tmp/filecache:/tmp/filecache

UPDATE: Ahhhh, this doesn’t work. This creates a volume inside: /volumes/<myapp>/tmp/filecache

So it looks like I don’t have the option to mount a volume outside the /volumes directory.