I have a sitemap-index.xml with 7 files. After updating one of the 7 with new content should I ping Google with the index file or with the updated file?
In my index I also use lastmod so Google can see when downloading it which item is newest.
The Google help page on submitting sitemaps says:
"Note: If you are providing a Sitemap index file, you only need to issue a single HTTP request that includes the location of the Sitemap index file; you don't need to issue individual requests for each Sitemap listed in the index."
Assuming that you're indeed providing correct
lastmod timestamps in your index, I'd assume that Googlebot would be smart enough not to reload the sitemaps that haven't changed.
If you really want to be sure, I guess you could do it and later look in your access logs to see which sitemaps were actually fetched. If you do that, please report the results here so that we can have a definitive answer. :-)