MongoDB in Docker, running with QEMU for AVX(2) support anywhere

Some applications require MongoDB, and MongoDB builds since v5 require AVX extensions on x86-64 systems. One solution is to rebuild Mongo without AVX but as the workflow of authors of Mongo doesn't cater to turning this off, the maintenance burden of disabling AVX increases. Also, it takes forever to build on older systems, so if you're building it on the system that doesn't have AVX, chances are it'll take hours to build as well.

So, I present to you an even worse, but simpler solution: Use the official builds, but run them through qemu-x86_64-static, because since December 2022's version 7.2 of QEMU, AVX(2) instructions can be emulated.

This will be really slow in certain use cases, because QEMU is being used as a full emulator, not with an accellerated hypervisor. You'll be emulating x86-64 on x86-64. I don't know if that affords any speedup over doing the same on, say, ARM64, but I doubt it as it's a niche case.

However, for light workloads (like a tiny wireless deployment that uses a controller that needs Mongo), it should be fine.

How it works

All this image does is move the old mongo binaries from the official image to new files, and replace their original names with scripts that wrap them in qemu-x86_64-static.

How to build

You'll need to provide qemu-x86_64-static, which on Debian, for example, is available through the qemu-user-static package. The multiarch/qemu-user-static images on Docker Hub don't include their own architecture's emulator (i.e. x86-64 isn't added on x86-64 systems), else I would use that.

The Dockerfile uses Debian's qemu-user-static package to source qemu-x86_64-static and drop it into the modified mongo image. You don't need qemu installed on your host system when building.

Building is simple. To create the image mongo-qemu:latest:

docker build -t mongo-qemu:latest .

You can then use it locally, or push it to a registry.

Mongo versions

The default mongo version is 7.0 but you can specify the build arg mongo_tag to select another, e.g.:

docker build -t mongo-qemu:latest --build-arg mongo_tag=6.0 .

How to use

Refer to the newly built container by tag, or incorporate it into your docker-compose.yml file.

Notes

As barely anything is changed from the official container, the entrypoint script will still warn you about lack of AVX support. However, the script doesn't error at this point, instead leaving it to the mongod binary to crash with a SIGILL illegal instruction. Of course, thanks to QEMU, that doesn't happen.