Managing your crawl budget: If your site is particularly large (hundreds of millions of pages that change periodically, or perhaps tens of millions of pages that change frequently), Google might not be able to crawl your entire site as often as you'd like, so you might need to point Google to the most important pages on your site. The best mechanism for doing so at present is to list your most recently updated or most important pages in your sitemaps, and hiding your less important pages using robots.txt rules.