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Hi, I am Bruno Paulino.
Software is my craft.

Fine tuning self-hosted Gitlab server to solve SSH scaling problems

July 5, 2020

At my current job, we have a self-hosted Giltab instance to control our codebases and run automation jobs. If you don’t know what Gitlab is, you probably know Github which is a platform to work in collaboration with software teams using Git for version control.

We recently onboarded a new partner to support us with our ERP system. We are working together to improve our current deployment pipeline and make it more resilient and robust. We use Gitlab CI to automate our workflows and make our codebase more reliable.

After setting up more runners, we noticed something strange happening randomly and quite frequently during our CI builds. While fetching more repositories, some builds were breaking with the following message:

fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.

Humm, that seems strange. Our runners have the correct access rights to the repositories. Also, this error was happening randomly for different repositories that clearly working during other exact same builds.

With this information at hand, we started off the debugging journey. We started monitoring the system and checking the server logs. Everything seemed normal. There was nothing draining our resources to the point where our server would start dropping connections.

SSH Load Balancing and MaxStartups

Discussing the issue among my smart coworkers, we couldn’t see a reason for Gitlab itself to be dropping connections with the amount of workload. There could be some configuration with our OpenSSH defaults that might be responsible for that. And we indeed found one.

OpenSSH has this neat setting called MaxStartups which is composed by 3 values divided by columns represented as start:rate:full. They mean:

  • start: The max number of unauthenticated connections
  • rate: The percentage of connections that will be dropped once our total of unauthenticated connections reaches the number specified at start.
  • full: By the time the queue of pending connections reaches the number specified at full, all connections will be dropped.

If you look at /etc/ssh/sshd_config, the SSH configuration file, you will see:

MaxStartups 10:30:100

This configuration means: starting with 10 pending to authenticate connections, our server will start dropping 30% of new connection attempts. Once our queue of pending connections reaches 100, all new connections will be dropped with no mercy.

After understanding how MaxStartups work, we looked again at our runners and we noticed that some jobs were fetching 10 different repositories in parallel during the build and consolidate them to build a Docker image. We were really playing the dice when running those builds 😅.

With that in mind, we tweaked MaxStartups with the following values:

MaxStartups 60:60:300

After that, just restart our SSH daemon with:

systemctl restart ssh.service

We could see that the MaxStartups change instantly fixed the issue. Our builds were flying again with zero connection drops. The new settings allowed us greater leeway to connect to our server simultaneously, reducing the risk of dropped connections.

Defaults can take you only so far

The default settings were more than enough for us to start using Gitlab with its Omnibus package. I didn’t even know about those SSH settings to begin with, but once you start hitting scaling problems, there is usually a escape hatch that can help you to leverage the same resources to a much greater usage scenario.

Only after fixing the issue that I found Gitlab themselves had the exact same issue. They wrote a great blog post about it.