I'm planning a huge gog migration from internal to external HD. So, I was thinking of checking if all my stuff is uptodate beforehands. The only method I know is through the gog downloader and the gog game catalogue page : manually re-downloading everything, and letting the downloader "skip" what is already there.

Like, you know, sometimes, when you already have it, instead of re-downlading it bit by bit, the downloader kind of strides through it with giant 50 Mb steps.

Sometimes.

Cause, some other times, it DOES re-download it all, bit by bit, even though there is no change in the version.

And it takes a lot of time and it is very annoying. Like now, re-downloading the whole Jade Empire, just because. Jade Empire is a big Jade Empire.

Why does it do that ? Because gog changed some ad image in the installer, so the whole 6 Gb lot has to be updated ? Of because the downloader is totally random, and recognises versions only half of times ? Or because I sneeze ?

Has anyone an explanation of that mystery, and/or an alternative method for this global check ?

... And yeah, I do expect gog to ask me to re-download the whole collection next week for galaxy reasons, which adds to the irony, but, still, I've started this now, and middle through my collection, and I really need some internal HD space, so...

Thanks for the remainder, my IPS is reseting my usage tomorrow. Time to download stuff I guess.

Otherwise, I think (don't quote me), that GOG's downloader uses either the version number or the hash value of part of the file to determine whether to redownload and sometimes, GOG may silently update minor stuff without telling you to update (for example, updating installers from 1.x to 2.x was silent unless there was also updates in the game's content).

As far as I know, any change on an installer file causes a version number update (even if it's on the minor part of the version number), but I could be wrong. It's probably something you could verify yourself.

Overall, yeah, the downloader is unfortunately buggy. There are cases where I just turned it off and used the browser instead (yeah, the queue is limited, but it doesn't hang and waste my time or my bandwith like the GOG downloader sometime does).

PS: There are some free open source third party downloaders that interact with GOG's API to get your content if you are unhappy with the official downloader. There is one that is pretty popular, though I vaguely recall reading that its maintainer said he would stop updating in anticipation of Galaxy.

I've been wondering this as well. If the version number is exactly the same, what exactly is throwing off the CRC/Hash information?

I'm afraid this is the answer and it sucks. I recall Judas or a different staff member mentioning it somewhere.

Ok.

If it's so, it's indeed bad. Real bad. Hear that, GOG staff ? THIS IS REALLY BAD.

I have to cancel the whole procedure. It would make me re-download almost the whole collection for zero reason.

Gog, your system is NOT useful, there. It is not good.

My opinion on this is negative.

As in : it should not be like that (it should be DIFFERENT instead).

...and it's all because some games were removed from the catalog! Boooooo!

To be honest, they should be using a torrent based client like any sane store. It's not like it's impossible to only allow authorized downloads.

On the upside, if a file gets corrupted, you can download just the portion that went bad and can verify the integrity of your entire collection.

I'm not really sure why GOG doesn't do that, I know Blizzard uses a torrent like system and I know Humble offers torrents.

The downloader check uses the following algorithm:
1) See if file(s) with the specific filename exist in the folder. If they don't, download the full file.
2) If the file exist, verify its hash (10MB chunks). Any chunk that fails the hash check gets redownloaded.
3) If all chunks pass the hash check (whether the file on disk, or the one re-downloaded), proceed to next file.

There has been at least one incident where GOG used the same filename, but the file was smaller, which caused the downloader to keep an erroneously larger file (chunks 1-40 checked, chunk 41 ignored, yet still part of the file).

Now, if a file is different, either due to very silent update, or due to bit corruption, only the corrupt chunks will be redownloaded, even if the downloader claims to download the full file. The usual lack of distinction between "verifying" and "downloading".

P.S. If you have access to a linux machine, try the lgogdownloader for the verification.

Even if the OP doesn't one can always install lgogdownloader to a live thumbdrive and download from there. It's a bit more advanced technically, but better than using the really bad GOG approved downloader. I hope when the new client comes out that they make it work efficiently.

This explains why I've seen some speedy process (verification) turn to a crawl (redownload) mid-file. But then, it crawls down to the end, redownloading all the following part...