I have a couple of questions about the publishing process.
What happens if publishing fails half way through? is the site maintained in its previous state, or does it end up with half old and half new files?
Say a site uses 800MB of disk space. On my Mac, I made changes that affect 200MB, or 25%. In addition to the 800MB used by my own site, how much buffer space does Sparkle need while publishing? is it 200MB, 800MB, or nothing?
Sparkle attempts to mitigate the possibility of a broken/incomplete website and to date we have actually had no report of that happening.
The way it’s done is all brand new files are published in their final place, but all files replacing existing ones are published to a “sparkle_upload” folder. Finally all replacement files are moved in their final place, and no longer used files are deleted.
In practice images are always generated with a new name if their content is changed (this is for unified cache coherence from within Sparkle’s compressed image cache all the way to the user browser), so images always go to their final place. Smaller css/js files are also revved (in this case to prevent browser caching not reloading them), and end up in their final place. html files got to the sparkle_upload folder and are moved into place around the end of the publishing process.
So unless that last rename step fails or is interrupted, it’s either the old site or the new site.
I’m not sure what you mean with buffer space, but because of the technique above, 200MB of new content will sit along side the old content until publishing completes, so the total space used would peak at 1000MB, and go back down to 800MB when the publishing finalization clears the now unused files.
Nice to know how Sparkle works on the server regards to publishing! 
Thank you Duncan, I appreciate the details.