When I rename a file using Mercurial, and then commit without any changes, why does it still send the full file to the repository? (I can tell because the subsequent push to the remote repository shows how much data is being transferred). Isn't it obvious to it that it simply needs a rename?
I'm using the latest version of TortoiseHG under Windows, and the file in question is a 20MB text file.
This is a known deficiency in the storage format used by Mercurial. You can search for "lightweight copies" for the full story, but briefly, the problem is that a new revlog is created for the new file name when you rename. The new revlog starts with a compressed snapshot of the full file — this is normally not a big problem, but it's still bigger than a zero-sized delta.
There's little you can do about it now unless you want to patch your Mercurial and run experimental code. The good news is that you just have to wait: the patches that we've been working on will be able to convert your existing repository into a more space efficient one automatically. This will happen when you hg clone over the network or if you use hg clone --pull locally.