Recently, I’ve had to troubleshoot some build errors during deployments with Heroku. The Heroku build was getting bigger with every release and we were seeing errors such as:
Warning: Your slug size (446 MB) exceeds our soft limit (300 MB) which may affect boot time.
I had to regularly clear the build cache:
heroku plugins:install heroku-builds heroku builds:cache:purge -a example-app
As I was unable to find information online about something like this, I asked on StackOverflow if anyone knew how to debug Heroku build sizes. It turns out there is a way to download the complete Heroku “slug”, that is to say the package that Heroku pushes to servers.
Here are some useful commands:
# list latest Heroku builds heroku slugs -a <app_name> | head # downloads a build heroku slugs download -a <app_name> <slug_id> # show directory sizes du -sch * # shows filenames that changed diff -qr <dir1> <dir2>