Earlier quoted context omitted.
> But I could duplicate the entire thing in about a day with inotify and rsync. The scaling is mostly on Amazon. No, you could duplicate a narrow piece of Dropbox's functionality in a day. You could not even get anywhere near duplicating the product and the service in anything less than months.
Dropbox pretty much uses librsync and, I believe, inotify on Linux. With Amazon S3. That is Dropbox. Literally. Polishing any product takes time. But that's universal and has nothing to do with engineering. What exactly is non-trivial about Dropbox?
1. Version control.
2. LAN sync.
3. Dropbox probably also does a lot to try and minimize their S3 costs - so probably a good deal of compression/duplication-detection on the backend.
4. Any and all networking issues. Even if using OSS gets you 98% of the way, Dropbox probably gets you 100% of the way.
5. Performance. I don't know what Dropbox is doing, but about a year ago I tested out Google Drive, SkyDrive, Dropbox, and SugarSync. I didn't want to use Dropbox because it gives you the least free storage, but Dropbox was consistently 2x faster than the competition (without using LAN sync). That makes a huge difference so I switched to it, and if Dropbox engineers are beating out Google and Microsoft engineers, they're clearly doing something special.