Prometheus 2.12, query logging, and startup failures on macos

Prometheus v2.12 added active query logging. The basic idea is that there is a mmaped JSON file that contains all of the queries currently running. If prometheus was to crash, that file would therefore be a list of the queries running at the time of the crash. Overall, not a bad idea.

Some friends had recently added prometheus to their development environments. This is wired up to grafana dashboards for their microservices, and prometheus is configured to store 14 days worth of time series data via a persistent volume from the developer desktops. We did this because it is valuable for the developers to be able to see the history of metrics before and after their changes.

Now we have a developer using macos as their primary development platform, and since prometheus 2.12 it hasn’t worked. Specifically this developer is using parallels to provide the docker virtual machine on his mac. You can summarise the startup for prometheus in the dev environment like this:

$ docker run ...stuff...
...snip...
level=error ts=2019-09-15T02:20:23.520Z caller=query_logger.go:94 component=activeQueryTracker msg="Failed to mmap" file=/prometheus-data/data/queries.active Attemptedsize=20001 err="invalid argument"
panic: Unable to create mmap-ed active query log

goroutine 1 [running]:
github.com/prometheus/prometheus/promql.NewActiveQueryTracker(0x7fff9917af38, 0x15, 0x14, 0x2a6b7c0, 0xc00003c7e0, 0x2a6b7c0)
	/app/promql/query_logger.go:112 +0x4d2
main.main()
	/app/cmd/prometheus/main.go:361 +0x52bd

And here’s the underlying problem — because of the way the persistent data is mapped into this container (via parallels sharing in this case), the mmap of the active queries file fails and prometheus fails to start.

In other words, since prometheus 2.12 your prometheus data files have to be stored on a filesystem which supports mmap. Additionally, there is no flag to just disable the active query logger.

So how do we work around this? Well, here’s a horrible workaround — in the data directory that is volume mapped into the container, create a symlink that is to a path that is mmapable inside the docker container, even if that path doesn’t exist outside the container. For example, given that we store the prometheus time series at $CONFIG/prometheus-data:

$ ln -s /tmp/queries.active "$CONFIG/prometheus-data/queries.active"

Note that /tmp/queries.active does not exist on the developer’s mac. Prometheus now starts and its puppies and kittens the whole way down.