Steam partially restarting game download

I was downloading Dota 2. I stopped the download at 4 GB but when I tried to restart the download next day it only showed 2.7 GB out of 4 GB. Does anyone know why this happened?

Solution 1:

@MacMan is correct. I can't suggest edits yet though, and I think this would be multiple comments which I don't want to get in trouble for as I'm still a noob here.

Further explanation:

Steam compresses downloads, which is why the download size of most games is less than the installation size. This also means that Steam has to not only download the data but then also decode it before it can be stored in the Steam/steamapps/common/{game name}folder. Steam also seems to download the data in chunks; this is most easily noticeable when it stops downloading for a while even though it's not finished, but yet it's still writing a lot of data to the disk during that time.

I'm not sure exactly how Steam's internal algorithm works, but I think that if there were any incomplete files they were likely discarded in favor of a fresh copy. It probably compares the local SHA or other message digest algorithm for each file to the known-good values from the server's copy of them and uses that as the deciding factor, as with many other tools meant to download other things including executable binaries which you really do not want to have incorrect data/signature.

As MacMan said though if you pause it but then give Steam time to finish saving what it's already downloaded you should have minimal -- if any -- data loss.