How do steam downloads work?

[SilkySmooth]

I was downloading the new atomic heart DLC when it stopped because I didn't have enough space (even though I did). I had to uninstall another game to clear up enough space for the download to continue. I saw +80gb in the download page for the DLC yet when it finished downloading only 3.7gb of data was downloaded and that was the full size of the DLC.

The download page next to the cancel button looked something like this: "Downloaded: 3.7gb / (idk) +80gb". why did I need more space that what the download needed for the download to work. I never thought this would be an issue but it seems completely stupid.

[Cathulhu]

There is the patch size. That is the smaller number.
Then there is the size of data that will be affected, that is the bigger number.

Downloads are compressed and encrypted, processing that data needs some space.
Additionally, all files that are going to be updated/patched are copied. So if a large amount of data is going to be affected by an update, you need a large amount of free space, temporarily, to do so.

[DarkCrystalMethod]

Steam downloads the installer(or the patching program) and then once its downloaded runs the installer. You'd need the size of the download plus 'some' extra room for it to work its magic.
The installer/patcher contains a mix of the raw files and those that get generated, and perhaps some scripts to synchronize your settings with the Steam Cloud once its 'done'.
The specifics are intentionally not revealed so most end users will not ever know for sure whats in the download itself. Also their mechanism changed and they called their newer download mechanism 'steam pipe'.

[SilkySmooth]

Alright then. Thanks for the clear ups guys.

[NakiBest]

Get a bigger and/or 2nd SSD/HDD. :-)

[aiusepsi]

This is really glossing over the details, but broadly, what Steam does is only downloads the difference (or 'delta') between the old versions of the game files and the new versions. It then constructs each updated file by copying across unchanged bits from the old versions of files, and downloading the new bits.

While the download is happening, the new updated files that are being constructed and the old files which will be replaced temporarily coexist, so you temporarily need a little extra space while the download is happening. Once the download is done, the old files are erased and replaced with the new files.

And onto what the other posters got wrong:

Cathulhu の投稿を引用：

Downloads are compressed and encrypted, processing that data needs some space.

Decompression and decryption all happens in memory (i.e. RAM), no extra disk space is required.

DarkCrystalMethod の投稿を引用：

Steam downloads the installer(or the patching program) and then once its downloaded runs the installer.

That is not what it does.

The installer/patcher contains a mix of the raw files and those that get generated, and perhaps some scripts to synchronize your settings with the Steam Cloud once its 'done'.

No. There's not an installer. Steam has manifests which specify how the files are constructed out of blocks, it compares the manifests for the old and new versions to work out which blocks it already has, and which it needs to download. It grabs those blocks individually from the content servers; there's no monolithic installer.

The specifics are intentionally not revealed so most end users will not ever know for sure whats in the download itself.

People have reverse-engineered what it does, there's at least one third-party reimplementation of the download system.

Also their mechanism changed and they called their newer download mechanism 'steam pipe'.

For reference, the "newer" mechanism is still about ten years old at this point.