Last modified: 2013-06-18 14:45:10 UTC
It's not necessary to regenerate sitemap files if it didn't actually change. This might not make sense for Wikipedia where something changes for sure within a day, but might be quite useful for smaller sites. One simple solution (considering current code structure) would be to generate files into temporary folder simultaneously calculating last modified timestamps and only moving new files in place of old ones if they actually have new entries (calculated timespamps are newer then old file's timestamps). Might also be a good idea to change file's timestamp to match calculated last modified timestamp. Index sitemap should also be created using these last modified timestamps.
All I know is I use a cronjob to generate the sitemaps every month. Well, indeed for small wikis, if no page at all in a namespace changes, then that sitemap.gz for that namespace does not need to be replaced. Anyway, Google etc. will still see the date of the individual files are the same as last time. So though the idea is worthy, the savings aren't very big.
On a small wiki the sitemap is fast to produce, I don't think there is any point in saving a few CPU cycles by adding some more complexity.
