It's an impressive feat but something I find even more impressive is the scale they need to be prepared for. The performance challenges of maintaining infrastructure for as many developers as Facebook/Google employ is constant and demanding.

At my extremely small startup in the past ~6 months I've made about 1,000 files (@ HEAD, not including deletions). So, maybe, at the absolute worst case each engineer could produce ~150 files/month. Facebook likely has ~1,000 engineers. That's a worst case 1,800,000 files/year + an existing massive code base.

Good luck to the engineers who are forever battling the scaling issues underlying these orgs! It's extremely impressive to even hear about overcoming these huge hurdles.

It all breaks down when it does not because watchman decides to come to a halt and basic arc pull takes more than 10mins after a week of no action on the repo :)